Semiconductor device, display device, and electronic device

ABSTRACT

A load, a transistor which controls a current value supplied to the load, a capacitor, a power supply line, and first to third switches are provided. After a threshold voltage of the transistor is held by the capacitor, a potential in accordance with a video signal is inputted and a voltage that is the sum of the threshold voltage and the potential is held. Accordingly, variation in current value caused by variation in threshold voltage of the transistor can be suppressed. Therefore, a desired current can be supplied to a load such as a light emitting element. In addition, a display device with a high duty ratio can be provided by changing a potential of the power supply line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/541,255, filed Nov. 14, 2014, now allowed, which is a continuation ofU.S. application Ser. No. 13/691,286, filed Nov. 30, 2012, now U.S. Pat.No. 8,890,180, which is a continuation of U.S. application Ser. No.11/562,678, filed Nov. 22, 2006, now U.S. Pat. No. 8,325,111, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2005-349165 on Dec. 2, 2005, all of which are incorporated byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device functioning tocontrol a current supplied to a load by a transistor, and a displaydevice including a pixel formed using a current-drive display element ofwhich luminance changes in accordance with a signal, and a signal linedriver circuit and a scan line driver circuit which drive the pixel. Thepresent invention also relates to a driving method thereof. Further, thepresent invention relates to an electronic device having the displaydevice in a display portion.

2. Description of the Related Art

In recent years, a self-luminous display device using a light emittingelement such as an electroluminescent (EL) element in a pixel, aso-called light emitting device has attracted attention. As a lightemitting element used for such a self-luminous display device, anorganic light emitting diode (OLED) and an EL element have attractedattention, and have been used for an EL display or the like. Since theselight emitting elements emit light by themselves, they have advantagessuch as higher pixel visibility, no backlight required, and higherresponse speed, over a liquid crystal display. Note that the luminanceof many of light emitting elements is controlled by the value of currentflowing to the light emitting element.

In addition, development of an active matrix display device in whicheach pixel is provided with a transistor that controls light emission ofa light emitting element has been advanced. The active matrix displaydevice is expected to be put into practical use because not only can itachieve high-definition and large-screen display that is difficult for apassive matrix display device, but also it operates with less powerconsumption than a passive matrix display device.

A structure of a pixel of a conventional active matrix display device isshown in FIG. 46 (Reference 1: Japanese Published Patent Application No.H8-234683). The pixel shown in FIG. 46 includes thin film transistors(TFTs) 11 and 12, a capacitor 13, and a light emitting element 14, andis connected to a signal line 15 and a scan line 16. Note that either asource electrode or a drain electrode of the TFT 12 and one electrode ofthe capacitor 13 are supplied with a power supply potential Vdd, and anopposite electrode of the light emitting element 14 is supplied with aground potential.

At this time, when using amorphous silicon for a semiconductor layer ofthe TFT 12 which controls a current value supplied to the light emittingelement, that is, a drive TFT, a change in threshold voltage (Vth) iscaused by deterioration or the like. In this case, although the samepotential is applied to different pixels through the signal line 15,current flowing to the light emitting element 14 differs from pixel topixel and display luminance becomes nonuniform among pixels. Note thatalso in the case of using polysilicon for a semiconductor layer of adrive TFT, characteristics of the transistor are deteriorated or variedlikewise.

An operating method using a pixel of FIG. 47 is proposed in Reference 2to improve the above problem (Reference 2: Japanese Published PatentApplication No. 2004-295131). The pixel shown in FIG. 47 includes atransistor 21, a drive transistor 22 which controls a current valuesupplied to a light emitting element 24, a capacitor 23, and the lightemitting element 24, and the pixel is connected to a signal line 25 anda scan line 26. Note that the drive transistor 22 is an NMOS transistor;either a source electrode or a drain electrode of the drive transistor22 is supplied with a ground potential; and an opposite electrode of thelight emitting element 24 is supplied with Vca.

A timing chart showing the operation of this pixel is shown in FIG. 48.In FIG. 48, one frame period is divided into an initialization period31, a threshold (Vth) write period 32, a data write period 33, and alight emitting period 34. Note that the one frame period corresponds toa period for displaying an image for one screen, and the initializationperiod, the threshold (Vth) write period, and the data write period arecollectively referred to as an address period.

First, in the threshold write period 32, a threshold voltage of thedrive transistor 22 is written into the capacitor. After that, in thedata write period 33, a data voltage (Vdata) showing a luminance of thepixel is written into the capacitor, and thus Vdata+Vth is accumulatedin the capacitor. Then, in the light emitting period 34, the drivetransistor 22 is turned on, so that the light emitting element 24 emitslight at a luminance specified by the data voltage by changing Vca. Suchoperation reduces a variation in luminance due to fluctuation inthreshold voltage of a drive transistor.

Reference 3 also discloses that a voltage corresponding to the sum of adata potential and a threshold voltage of a drive TFT is a gate-sourcevoltage and current flowing to the TFT does not change even when thethreshold voltage of the TFT is changed (Reference 3: Japanese PublishedPatent Application No. 2004-280059).

In either of the operating methods described in References 2 and 3, theinitialization, the writing of a threshold voltage, and the lightemission described above are performed by changing a potential Vcaseveral times in each one frame period. In these pixels, one electrodeof a light emitting element to which Vca is supplied, that is, anopposite electrode is formed entirely over a pixel region. Therefore,the light emitting element cannot emit light if there is even a singlepixel which performs data writing operation besides initialization andwriting of a threshold voltage. Thus, a ratio of a light emitting periodto one frame period (i.e. a duty ratio) is lowered as shown in FIG. 49.

A low duty ratio requires a high current value supplied to the lightemitting element or a drive transistor, which results in increases of avoltage applied to the light emitting element and power consumption. Inaddition, the light emitting element or the drive transistor becomeseasily deteriorated; therefore, much more power is required to obtain aluminance equivalent to that before deterioration.

Further, since the opposite electrode is connected to all pixels, thelight emitting element functions as an element with large capacitance.Therefore, more power needs to be consumed to change the potential ofthe opposite electrode.

SUMMARY OF THE INVENTION

In view of the above problems, it is an object of the present inventionto provide a display device which consumes less power and has a highduty ratio. It is another object of the present invention to obtain apixel structure, a semiconductor device, and a display device withlittle deviation of luminance from that specified by a data potential.

Note that the scope of the present invention is not limited only to adisplay device having a light emitting element, and it is an object ofthe present invention to suppress variation in current value caused byvariation in threshold voltage of a transistor. Therefore, a destinationsupplied with current controlled by a drive transistor is not limited tothe light emitting element.

One aspect of the present invention provides a semiconductor devicehaving a pixel including a transistor, a first switch, and a secondswitch, in which one of either a source electrode or a drain electrodeof the transistor is electrically connected to a gate electrode of thetransistor through the first switch, the other of either the sourceelectrode or the drain electrode of the transistor is electricallyconnected to a pixel electrode, the other of either the source electrodeor the drain electrode of the transistor is electrically connected tothe second switch, and a signal in accordance with a gray scale level ofthe pixel is inputted to the gate electrode of the transistor.

One aspect of the present invention provides a semiconductor deviceincluding a storage capacitor, a transistor, a first switch, a secondswitch, and a third switch, in which one of either a source electrode ora drain electrode of the transistor is electrically connected to a firstwiring, the other of either the source electrode or the drain electrodeof the transistor is electrically connected to a pixel electrode, theother of either the source electrode or the drain electrode of thetransistor is electrically connected to a second wiring through thethird switch, a gate electrode of the transistor is electricallyconnected to a third wiring through the first switch, the gate electrodeof the transistor is electrically connected to the first wiring throughthe second switch, and the other of either the source electrode or thedrain electrode of the transistor is electrically connected to the gateelectrode through the storage capacitor.

One aspect of the present invention provides a semiconductor deviceincluding a capacitor, a transistor, a first switch, a second switch,and a third switch, in which one of either a source electrode or a drainelectrode of the transistor is electrically connected to a first wiring,the other of either the source electrode or the drain electrode of thetransistor is electrically connected to a pixel electrode, the other ofeither the source electrode or the drain electrode of the transistor iselectrically connected to a second wiring through the third switch, agate electrode of the transistor is electrically connected to a thirdwiring through the first switch, the gate electrode of the transistor iselectrically connected to the first wiring through the second switch,and the other of either the source electrode or the drain electrode ofthe transistor is electrically connected to the gate electrode throughthe capacitor.

One aspect of the present invention provides a semiconductor deviceincluding a transistor, a capacitor, a first switch, a second switch, athird switch, and a fourth switch, in which one of either a sourceelectrode or a drain electrode of the transistor is electricallyconnected to a first wiring through the fourth switch, the other ofeither the source electrode or the drain electrode of the transistor iselectrically connected to a pixel electrode, the other of either thesource electrode or the drain electrode of the transistor iselectrically connected to a second wiring through the third switch, agate electrode of the transistor is electrically connected to a thirdwiring through the first switch, the gate electrode of the transistor iselectrically connected to the first wiring through the second switch,and the other of either the source electrode or the drain electrode ofthe transistor is electrically connected to the gate electrode throughthe capacitor.

One aspect of the present invention provides a semiconductor deviceincluding a transistor, a capacitor, a first switch, a second switch, athird switch, and a fourth switch, in which one of either a sourceelectrode or a drain electrode of the transistor is electricallyconnected to a first wiring, the other of either the source electrode orthe drain electrode of the transistor is electrically connected to apixel electrode through the fourth switch, the other of either thesource electrode or the drain electrode of the transistor iselectrically connected to a second wiring through the fourth switch andthe third switch, a gate electrode of the transistor is electricallyconnected to a third wiring through the first switch, the gate electrodeof the transistor is electrically connected to the first wiring throughthe second switch, and the other of either the source electrode or thedrain electrode of the transistor is electrically connected to the gateelectrode through the fourth switch and the capacitor.

The second wiring may be the same as a wiring which controls the thirdswitch.

The second wiring may be any of scan lines which control first to thirdswitches of a preceding row or a subsequent row.

The transistor may be an n-channel transistor. Further, a semiconductorlayer of the transistor may be formed of a non-crystalline semiconductorfilm. Furthermore, the semiconductor layer of the transistor may beformed of amorphous silicon.

The semiconductor layer of the transistor may be formed of a crystallinesemiconductor film.

In the above invention, a potential inputted to the first wiring mayhave two values V1 and V2, the potential may be V2 when the first tothird switches are in an off state, V1 may be a potential higher than apotential inputted to the second wiring, the difference between V1 andV2 may be larger than a threshold voltage of the transistor, and V2 maybe higher than V1.

In addition, the transistor may be a p-channel transistor. In that case,in the above invention, a potential inputted to the first wiring mayhave two values V1 and V2, the potential may be V2 when the first tothird switches are in an off state, V1 may be a potential lower than apotential inputted to the second wiring, the difference between V and V2may be larger than an absolute value of a threshold voltage of thetransistor, and V2 may be lower than V1.

One aspect of the present invention provides a semiconductor deviceincluding a transistor of which one of either a source electrode or adrain electrode is electrically connected to a first wiring and theother of either the source electrode or the drain electrode iselectrically connected to a second wiring, a storage capacitor whichholds a gate-source voltage of the transistor, a means to hold a firstvoltage in the storage capacitor by applying a first potential which isinputted to the first wiring to a gate electrode of the transistor andapplying a second potential which is inputted to the second wiring tothe source electrode of the transistor, a means to discharge a voltageof the storage capacitor down to a second voltage, a means to hold afifth voltage that is the sum of the second voltage and a fourth voltagein the storage capacitor by applying a potential that is the sum of thefirst potential and a third voltage to the gate electrode of thetransistor, and a means to supply current set for the transistor to aload by inputting a third potential that is different from the firstpotential to the first wiring.

One aspect of the present invention provides a semiconductor deviceincluding a transistor in which one of either a source electrode or adrain electrode is electrically connected to a first wiring and theother of either the source electrode or the drain electrode iselectrically connected to a second wiring, a storage capacitor whichholds a gate-source voltage of the transistor, a means to hold a firstvoltage in the storage capacitor by applying a first potential which isinputted to the first wiring to a gate electrode of the transistor andapplying a second potential inputted to the second wiring to the sourceelectrode of the transistor, a means to discharge a voltage of thestorage capacitor down to a threshold voltage of the transistor, a meansto hold a fourth voltage that is the sum of the threshold voltage of thetransistor and a third voltage by applying a potential that is the sumof the first potential and a second voltage to the gate electrode of thetransistor, and a means to supply current set for the transistor to aload by inputting a third potential that is different from the firstpotential to the first wiring.

The transistor may be an n-channel transistor. Further, a semiconductorlayer of the transistor may be formed of a non-crystalline semiconductorfilm. Furthermore, the semiconductor layer of the transistor may beformed of amorphous silicon.

Alternatively, the semiconductor layer of the transistor may be formedof a crystalline semiconductor film.

In the above invention, the first potential may be a potential higherthan the second potential, the difference between the first potentialand the second potential may be larger than a threshold voltage of thetransistor, and the first potential may be lower than the thirdpotential.

Furthermore, the transistor may be a p-channel transistor. In this case,in the present invention, the first potential may be a potential lowerthan the second potential, the difference between the first potentialand the second potential may be larger than the absolute value of thethreshold voltage of the transistor, and the first potential may behigher than the third potential.

One aspect of the present invention provides a display device includingthe above-described semiconductor device. In addition, it also providesan electronic device having the display device in a display portion.

Note that the switch described in this specification is not particularlylimited and may be an electrical switch or a mechanical switch as longas it can control current flow. The switch may be a transistor, a diode,or a logic circuit that is a combination thereof. In the case of using atransistor as the switch, the transistor operates as a mere switch.Therefore, the polarity (conductivity type) of the transistor is notparticularly limited. However, it is desirable to use a transistorhaving a polarity with lower off-current. As the transistor with lowoff-current, a transistor provided with an LDD region, a transistorhaving a multi-gate structure, or the like can be used. In addition, itis desirable to use an n-channel transistor when a transistor to beoperated as a switch operates in a state where the potential of a sourceelectrode thereof is close to a lower potential side power source (suchas Vss, GND, or 0 V), whereas it is desirable to use a p-channeltransistor when the transistor operates in a state where the potentialof a source electrode thereof is close to a higher potential side powersource (such as Vdd). This is because the absolute value of agate-source voltage can be increased, so that the transistor easilyoperates as a switch. Note that the switch may be of CMOS type usingboth an n-channel transistor and a p-channel transistor.

Note that the phrase “being connected” in the present invention issynonymous with being electrically connected. Thus, another element,switch, or the like may be interposed.

Note that the load may be anything. For example, a display medium ofwhich contrast varies by an electromagnetic action, such as a liquidcrystal element or electronic ink can be used as well as a lightemitting element such as an EL element (an organic EL element, aninorganic EL element, or an EL element containing an organic materialand an inorganic material) or an electron emitting element. Note that afield emission display (FED), an SED flat-panel display (SED:Surface-conduction Electron-emitter Display), or the like can be givenas an example of a display device using an electron emitting element. Inaddition, electronic paper can be given as a display device usingelectronic ink.

In the present invention, there is no limitation on the kind ofapplicable transistors, and a thin film transistor (TFT) using anon-single crystal semiconductor film typified by an amorphous siliconfilm or a polycrystalline silicon film, a transistor formed using asemiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, or a bipolar transistor, a transistor using anorganic semiconductor or a carbon nanotube, or another transistor can beused. In addition, there is no limitation on the kind of substrate wherethe transistor is located, and the transistor can be located over asingle crystalline substrate, an SOI substrate, a glass substrate, aplastic substrate, or the like.

Note that as described above, the transistor in the present inventionmay be of any type and may be formed over any type of substrate.Accordingly, all circuits may be formed over a glass substrate, aplastic substrate, a single crystalline substrate, an SOI substrate, orany other substrates. Alternatively, a part of the circuits may beformed over a substrate, and another part of the circuits may be formedover another substrate. In other words, all of the circuits are notnecessarily formed over the same substrate. For example, a part of thecircuits may be formed over a glass substrate using TFTs, another partof the circuits may be formed as an IC chip over a single crystallinesubstrate, and the IC chip may be connected onto the glass substrate byCOG (Chip On Glass). Alternatively, the IC chip may be connected to theglass substrate by TAB (Tape Automated Bonding) or using a printedcircuit board.

In this specification, one pixel means a color element. Accordingly, inthe case of a full-color display device including R (red), G (green),and B (blue) color elements, one pixel means any one of R, Q and B colorelements.

Note that the phrase “pixels are arranged in matrix” in thisspecification includes the case where when full-color display isperformed with three color elements (e.g. RGB), pixels of three colorelements constituting the smallest unit of an image are arranged in aso-called delta pattern as well as the case where pixels are arranged ina grid pattern formed by a combination of vertical stripes andhorizontal stripes. In addition, the sizes of the pixels may bedifferent from color element to color element.

Note that the term “semiconductor device” in this specification means adevice including a circuit including a semiconductor element (such as atransistor or a diode). In addition, the term “display device” includesnot only a main body of a display panel in which a plurality of pixelsincluding a load and a peripheral driver circuit for driving the pixelsare formed over a substrate but also a display panel with a flexibleprinted circuit (FPC) or a printed wiring board (PWB) attached thereto.

According to the present invention, variation in current value caused byvariation in threshold voltage of a transistor can be suppressed.Therefore, a desired current can be supplied to a load such as a lightemitting element. In particular, when a light emitting element is usedas a load, a display device with less luminance variation and a highduty ratio can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a pixel structure described inEmbodiment Mode 1.

FIG. 2 is a timing chart for explaining operation of the pixel shown inFIG. 1.

FIGS. 3A to 3D are diagrams illustrating operation of the pixel shown inFIG. 1.

FIG. 4 is a diagram illustrating a pixel structure described inEmbodiment Mode 1.

FIG. 5 is a model diagram of a voltage-current characteristic inaccordance with channel length modulation.

FIG. 6 is a diagram illustrating a display device described inEmbodiment Mode 1.

FIG. 7 is a diagram illustrating a pixel structure described inEmbodiment Mode 3.

FIG. 8 is a diagram illustrating a pixel structure described inEmbodiment Mode 3.

FIG. 9 is a diagram illustrating a pixel structure described inEmbodiment Mode 3.

FIG. 10 is a diagram illustrating a pixel structure described inEmbodiment Mode 4.

FIG. 11 is a diagram illustrating a pixel structure described inEmbodiment Mode 4.

FIG. 12 is a diagram illustrating a pixel structure described inEmbodiment Mode 5.

FIG. 13 is a timing chart for explaining operation of the pixel shown inFIG. 12.

FIG. 14 is a diagram illustrating a pixel structure described inEmbodiment Mode 7.

FIG. 15 is a timing chart for explaining operation of the pixel shown inFIG. 14.

FIGS. 16A to 16D are diagrams illustrating operation of the pixel shownin FIG. 14.

FIG. 17 is a fragmentary sectional view of a pixel described inEmbodiment Mode 8.

FIGS. 18A and 18B are diagrams illustrating light emitting elementsdescribed in Embodiment Mode 8.

FIGS. 19A to 19C are diagrams illustrating extraction directions oflight described in Embodiment Mode 8.

FIGS. 20A and 20B are fragmentary sectional views of pixels described inEmbodiment Mode 8.

FIGS. 21A and 21B are fragmentary sectional views of pixels described inEmbodiment Mode 8.

FIGS. 22A and 22B are fragmentary sectional views of pixels described inEmbodiment Mode 8.

FIG. 23 is a fragmentary sectional view of a pixel described inEmbodiment Mode 8.

FIG. 24 is a fragmentary sectional view of a pixel described inEmbodiment Mode 8.

FIGS. 25A and 25B are diagrams illustrating a display device describedin Embodiment Mode 9.

FIGS. 26A and 26B are diagrams illustrating display devices described inEmbodiment Mode 9.

FIGS. 27A and 27B are diagrams illustrating display devices described inEmbodiment Mode 9.

FIG. 28 is a fragmentary sectional view of a pixel described inEmbodiment Mode 9.

FIG. 29 is a diagram illustrating a pixel structure described inEmbodiment Mode 6.

FIG. 30 is a diagram illustrating a pixel structure described inEmbodiment Mode 6.

FIG. 31 is a diagram illustrating a pixel structure described inEmbodiment Mode 6.

FIG. 32 is a diagram illustrating a pixel structure described inEmbodiment Mode 7.

FIGS. 33A to 33H are diagrams illustrating electronic devices to whichthe present invention can be applied.

FIG. 34 is a diagram showing a configuration example of a cellularphone.

FIG. 35 is a diagram showing an example of an EL module.

FIG. 36 is a block diagram showing main components of an EL televisionreceiver.

FIG. 37 is a diagram illustrating a pixel structure described inEmbodiment Mode 6.

FIG. 38 is a top view of the pixel shown in FIG. 4.

FIGS. 39A and 39B are diagrams illustrating a pixel structure describedin Embodiment Mode 2.

FIG. 40 is a diagram illustrating write operation of a display devicedescribed in Embodiment Mode 1.

FIG. 41 is a diagram illustrating a pixel stricture described inEmbodiment Mode 5.

FIG. 42 is a diagram illustrating a driving method which combines adigital gray scale method and a time gray scale method.

FIG. 43 is a diagram illustrating a pixel structure described inEmbodiment Mode 6.

FIG. 44 is a diagram illustrating a pixel structure described inEmbodiment Mode 6.

FIG. 45 is a diagram illustrating a pixel structure described inEmbodiment Mode 1.

FIG. 46 is a diagram illustrating a pixel structure of conventional art.

FIG. 47 is a diagram illustrating a pixel structure of conventional art.

FIG. 48 is a timing chart for operating a pixel described in RelatedArt.

FIG. 49 is a diagram illustrating a ratio of a light emitting period toone frame period when using conventional art.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, modes of the present invention are explained. Note that itis easily understood by a person skilled in the art that the presentinvention can be embodied in many different modes and the modes anddetail of the present invention can be variously changed withoutdeviating from the spirit and the scope of the present invention.Therefore, the present invention is not interpreted as being limited tothe description of the mode. Note that the same reference numeral isused to denote the same component among the different drawings in thestructure of the present invention to be described below.

Embodiment Mode 1

A basic structure of a pixel of the present invention is explained withreference to FIG. 1. The pixel shown in FIG. 1 includes a transistor111, a first switch 112, a second switch 113, a third switch 114, acapacitor 115, and a light emitting element 116. Note that the pixel isconnected to a signal line 117, a first scan line 118, a second scanline 119, a third scan line 120, a power supply line 121, and apotential supply line 122. In this embodiment mode, the transistor IIIis an n-channel transistor and is turned on when a gate-source voltage(Vgs) thereof exceeds a threshold voltage (Vth). In addition, a pixelelectrode of the light emitting element 116 is an anode and an oppositeelectrode 123 is a cathode. Note that the gate-source voltage of thetransistor is referred to as Vgs; a drain-source voltage, Vds; athreshold voltage, Vth; a voltage accumulated in the capacitor, Vcs; andthe power supply line 121, the potential supply line 122, and the signalline 117 are also referred to as a first wiring, a second wiring, and athird wiring, respectively.

A first electrode (one of either a source electrode or a drainelectrode) of the transistor 111 is connected to the pixel electrode ofthe light emitting element 116; a second electrode (the other of eitherthe source electrode or the drain electrode) thereof is connected to thepower supply line 121; and a gate electrode thereof is connected to thepower supply line 121 through the second switch 113. In addition, thegate electrode of the transistor 111 is also connected to the signalline 117 through the first switch 112, and the first electrode thereofis also connected to the potential supply line 122 through the thirdswitch 114.

Further, the capacitor 115 is connected between the gate electrode andthe first electrode of the transistor 111. In other words, a firstelectrode of the capacitor 115 is connected to the gate electrode of thetransistor 111, and a second electrode of the capacitor 115 is connectedto the first electrode of the transistor 111. The capacitor 115 may beformed by sandwiching an insulating film between a wiring, asemiconductor layer, and an electrode or can be omitted by using a gatecapacitance of the transistor 111. Such a means to hold a voltage isreferred to as a storage capacitor.

Note that the first switch 112, the second switch 113, and the thirdswitch 114 are controlled to be turned on and off by inputting signalsto the first scan line 118, the second scan line 119, and the third scanline 120, respectively.

A signal in accordance with a gray scale level of the pixel whichcorresponds to a video signal, that is, a potential in accordance withluminance data is inputted to the signal line 117.

Next, operation of the pixel shown in FIG. 1 is explained with referenceto a timing chart in FIG. 2 and FIGS. 3A to 3D. Note that one frameperiod corresponding to a period for displaying an image for one screenis divided into an initialization period, a threshold write period, adata write period, and a light emitting period in FIG. 2. Theinitialization period, the threshold write period, and the data writeperiod are collectively referred to as an address period. The length ofone frame period is not particularly limited, but is preferably 1/60second or less so that an image viewer does not perceive flicker.

Note that a potential V1 is inputted to the opposite electrode 123 ofthe light emitting element 116 and a potential V1-Vth-α (α: an arbitrarypositive number) is inputted to the potential supply line 122. Inaddition, to the power supply line 121, the potential V1 is inputted inthe address period and a potential V2 is inputted in the light emittingperiod. Note that V2>V1.

Here, the potential of the opposite electrode 123 of the light emittingelement 116 is equal to the potential of the power supply line 121 inthe address period for the purpose of explaining operation. However,when the minimum potential difference which is necessary for the lightemitting element 116 to emit light is referred to as V_(EL), it isacceptable as long as the potential of the opposite electrode 123 ishigher than a potential V1-Vth-α-V_(EL). In addition, it is acceptableas long as the potential V2 of the power supply line 121 in the lightemitting period is higher than the sum of the potential of the oppositeelectrode 123 and the minimum potential difference (V_(EL)) which isnecessary for the light emitting element 116 to emit light. However,since the potential of the opposite electrode 123 is V1 here for thepurpose of explanation, it is acceptable as long as V2 is higher thanV1+V_(EL).

First, the first switch 112 is turned off and the second switch 113 andthe third switch 114 are turned on in the initialization period as shownin (A) in FIG. 2 and FIG. 3A. At this time, the first electrode of thetransistor 111 serves as a source electrode, and a potential thereof isequal to that of the potential supply line 122, which is V1-Vth-α. Onthe other hand, a potential of the gate electrode of the transistor 111is V1. Thus, the gate-source voltage Vgs of the transistor 111 is Vth+αand thus the transistor 111 is turned on. Then, Vth+α is held by thecapacitor 115 provided between the gate electrode and the firstelectrode of the transistor 111. In other words, it is acceptable aslong as the potential supply line 122 has such a potential as to turn onthe transistor 111 and the third switch 114 functions to select whetheror not to supply such a potential as to turn on the transistor 111 tothe first electrode.

Next, the third switch 114 is turned off in the threshold write periodshown in (B) in FIG. 2 and FIG. 3B. Therefore, the potential of thefirst electrode, i.e. the source electrode of the transistor 111 risesgradually and when it reaches V1-Vth, in other words, when thegate-source voltage Vgs of the transistor 111 reaches the thresholdvoltage (Vth), the transistor 111 is turned off. Thus, a voltage held bythe capacitor 115 is Vth.

In the subsequent data write period shown in (C) in FIG. 2 and FIG. 3C,the first switch 112 is turned on and a potential (V1+Vdata) inaccordance with luminance data is inputted from the signal line 117after turning off the second switch 113. At this time, the voltage Vcsheld by the capacitor 115 can be represented by Formula (1) wherecapacitances of the capacitor 115 and the light emitting element 116 arereferred to as C1 and C2, respectively.

$\begin{matrix}{{Vcs} = {{Vth} + {{Vdata} \times \frac{C\; 2}{{C\; 1} + {C\; 2}}}}} & (1)\end{matrix}$

Note that C2>>C1 because the light emitting element 116 is thinner andhas a larger electrode area than the capacitor 115. Thus, fromC2/(C1+C2), 1, the voltage Vcs held by the capacitor 115 is representedby Formula (2), and the transistor 111 is turned on. Note that when apotential Vdata≤0 is inputted, the transistor 111 can be turned off sothat the light emitting element 116 does not emit light

Vcs=Vth+Vdata  (2)

Next, in the light emitting period shown in (D) in FIG. 2 and FIG. 3D,the first switch 112 is turned off and the potential of the power supplyline 121 is set to V2. At this time, the gate-source voltage Vgs of thetransistor 111 is equal to Vth+Vdata, and current in accordance withthis Vgs flows to the transistor 111 and the light emitting element 116,so that the light emitting element 116 emits light.

Note that a current I flowing to the light emitting element isrepresented by Formula (3) when the transistor 111 is operated in thesaturation region.

$\begin{matrix}\begin{matrix}{I = {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{Vgs} - {Vth}} \right)}^{2}}} \\{= {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{Vth} + {Vdata} - {Vth}} \right)}^{2}}} \\{= {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}({Vdata})}^{2}}}\end{matrix} & (3)\end{matrix}$

A current I flowing to the light emitting element when the transistor111 is operated in the linear region is represented by Formula (4).

$\begin{matrix}\begin{matrix}{I = {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{Vgs} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} \\{= {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{Vth} + {Vdata} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} \\{= {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{({Vdata}){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}}\end{matrix} & (4)\end{matrix}$

Here, W denotes a channel width of the transistor 111; L, a channellength; μ, a mobility; and Cox, a storage capacitance.

According to Formulas (3) and (4), the current flowing to the lightemitting element 116 does not depend on the threshold voltage (Vth) ofthe transistor 111 regardless of whether the operation region of thetransistor 111 is either the saturation region or the linear region.Therefore, variation in current value caused by variation in thresholdvoltage of the transistor 11 can be suppressed and a current value inaccordance with luminance data can be supplied to the light emittingelement 116.

Accordingly, variation in luminance caused by variation in the thresholdvoltage of the transistor 111 can be suppressed. In addition, powerconsumption can be reduced because operation is performed with theopposite electrode fixed at a constant potential.

Furthermore, when the transistor 111 is operated in the saturationregion, variation in luminance due to deterioration of the lightemitting element 116 can also be suppressed. When the light emittingelement 116 is deteriorated, V_(EL) of the light emitting element 116increases and the potential of the first electrode, that is, the sourceelectrode of the transistor 111 rises. At this time, the sourceelectrode of the transistor 111 is connected to the second electrode ofthe capacitor 115; the gate electrode of the transistor 111 is connectedto the first electrode of the capacitor 115; and the gate electrode sideis in a floating state. Therefore, in accordance with the rise in thesource potential, the gate potential of the transistor 111 alsoincreases by the same amount as the rise in the source potential. Thus,Vgs of the transistor 111 does not change. Therefore, the currentflowing to the transistor 111 and the light emitting element 116 is notaffected even if the light emitting element is deteriorated. Note thatit is found also in Formula (3) that the current I flowing to the lightemitting element does not depend on the source potential and a drainpotential.

Therefore, when the transistor 111 is operated in the saturation region,variation in the threshold voltage of the transistor 111 and variationin the current flowing to the transistor 111 caused by deterioration ofthe light emitting element 116 can be suppressed.

Note that in the case where the transistor 111 is operated in thesaturation region, as the channel length L is shorter, a larger amountof current tends to flow when the drain voltage is significantlyincreased by avalanche breakdown.

When the drain voltage is increased to exceed a pinch off voltage, apinch off point moves to the source side and an effective channel lengthwhich substantially functions as a channel is reduced. This increases acurrent value. This phenomenon is referred to as channel lengthmodulation. Note that the pinch off point is a boundary portion at whichthe channel disappears and the thickness of the channel is 0 below thegate, and the pinch off voltage refers to a voltage when the pinch offpoint is at a drain edge. This phenomenon also occurs more easily as thechannel length L is shorter. For example, a model diagram of avoltage-current characteristic in accordance with channel lengthmodulation is shown in FIG. 5. Note that the channel lengths L of thetransistors (a), (b), (c) satisfy (a)>(b)>(c) in FIG. 5.

Accordingly, in the case of operating the transistor 11 l in thesaturation region, considering that an influence of deterioration of thelight emitting element 116 can be reduced as described above if thecurrent I is constant with respect to the drain-source voltage Vds, thecurrent I with respect to the drain-source voltage Vds is preferably asconstant as possible. Thus, the channel length L of the transistor 111is preferably longer. For example, the channel length L of thetransistor is preferably larger than the channel width W. In addition,the channel length L is preferably 10 μm to 50 μm inclusive and morepreferably 15 μm to 40 μm inclusive. However, the channel length L andthe channel width W are not limited thereto.

In addition, since a reverse bias voltage is applied to the lightemitting element 116 in the initialization period, a shorted portion inthe light emitting element can be insulated and deterioration of thelight emitting element can be suppressed. Thus, the lifetime of thelight emitting element can be extended.

Note that since the variation in current value caused by variation inthreshold voltage of the transistor can be suppressed, supplydestination of the current controlled by the transistor is notparticularly limited. Therefore, an EL element (an organic EL element,an inorganic EL element, or an EL element containing an organic materialand an inorganic material), an electron emitting element, a liquidcrystal element, electronic ink, or the like can be used as the lightemitting element 116 shown in FIG. 1.

In addition, it is acceptable as long as the transistor 111 functions tocontrol a current value supplied to the light emitting element 116, andthe kind of the transistor is not particularly limited. Therefore, athin film transistor (TFT) using a crystalline semiconductor film, athin film transistor using a non-single crystalline semiconductor filmtypified by an amorphous silicon film or a polycrystalline silicon film,a transistor formed using a semiconductor substrate or an SOI substrate,a MOS transistor, a junction transistor, a bipolar transistor, atransistor using an organic semiconductor or a carbon nanotube, oranother transistor can be used.

The first switch 112 selects timing to input a signal in accordance witha gray scale level of the pixel to the capacitor and controls a signalsupplied to the gate electrode of the transistor 111. The second switch113 selects timing to apply a predetermined potential to the gateelectrode of the transistor 111 and controls whether or not to supplythe predetermined potential to the gate electrode of the transistor 111.The third switch 114 selects timing to apply a predetermined potentialfor initializing a potential written in the capacitor 115 and decreasesthe potential of the first electrode of the transistor 111. Therefore,the first switch 112, the second switch 113, and the third switch 114are not particularly limited as long as they have the above functions.For example, each of the switches may be a transistor, a diode, or alogic circuit that is a combination thereof. Note that the first tothird switches are not particularly necessary if the signal or potentialcan be applied to the pixel at the above timing. For example, when thesignal in accordance with a gray scale level of the pixel can beinputted to the gate electrode of the transistor 111, the first switch112 does not need to be provided as shown in FIG. 45. A pixel shown inFIG. 45 includes a transistor 111, a second switch 113, a third switch114, and a pixel electrode 4540. A first electrode (one of either asource electrode or a drain electrode) of the transistor 111 isconnected to the pixel electrode 4540 and the third switch 114, and agate electrode of the transistor 111 is connected to a second electrodeof the transistor 111 through the second switch 113. Note that a gatecapacitance 4515 of the transistor 111 is used as a storage capacitor;therefore, the capacitor 115 in FIG. 1 does not particularly need to beprovided. Such a pixel can also suppress variation in current valuecaused by variation in threshold voltage of the transistor 111 byoperating each switch in accordance with the timing chart shown in FIG.2 and supplying a desired potential to each electrode. Thus, a desiredcurrent can be supplied to the pixel electrode 4540.

Next, the case of employing n-channel transistors as the first switch112, the second switch 113, and the third switch 114 is shown in FIG. 4.Note that a common reference numeral is shared with FIG. 1 to denote acommon component, and explanation thereof is omitted.

A first switching transistor 412 corresponds to the first switch 112; asecond switching transistor 413, the second switch 113; and a thirdswitching transistor 414, the third switch 114. Note that the channellength of the transistor 111 is preferably larger than that of any ofthe first switching transistor 412, the second switching transistor 413,and the third switching transistor 414.

A gate electrode of the first switching transistor 412 is connected to afirst scan line 118; a first electrode thereof, to a signal line 117;and a second electrode thereof, to a first electrode of a capacitor 115and a gate electrode of the transistor 111.

In addition, a gate electrode of the second switching transistor 413 isconnected to a second scan line 119; a first electrode thereof, to thefirst electrode of the capacitor 115 and the gate electrode of thetransistor 111; and a second electrode thereof, to a power supply line121 and a second electrode of the transistor 111.

A gate electrode of the third switching transistor 414 is connected to athird scan line 120; a first electrode thereof, to a second electrode ofthe capacitor 115, a first electrode of the transistor 111, and a pixelelectrode of a light emitting element 116; and a second electrodethereof, to a potential supply line 122.

Each switching transistor is turned on when a signal inputted to eachscan line is at an H level and turned off when the signal inputted is atan L level.

One mode of a top view of the pixel shown in FIG. 4 is shown in FIG. 38.A conductive layer 3810 includes a portion functioning as the first scanline 118 and the gate electrode of the first switching transistor 412,and a conductive layer 3811 includes a portion functioning as the signalline 117 and the first electrode of the first switching transistor 412.In addition, a conductive layer 3812 includes a portion functioning asthe second electrode of the first switching transistor 412, a portionfunctioning as the first electrode of the capacitor 115, and a portionfunctioning as the first electrode of the second switching transistor413. A conductive layer 3813 includes a portion functioning as the gateelectrode of the second switching transistor 413 and is connected to thesecond scan line 119 through a wiring 3814. A conductive layer 3822includes a portion functioning as the second electrode of the secondswitching transistor 413 and a portion functioning as the secondelectrode of the transistor 111 and is connected to the power supplyline 121 through a wiring 3815. A conductive layer 3816 includes aportion functioning as the first electrode of the transistor 111 and isconnected to a pixel electrode 3844 of the light emitting element. Aconductive layer 3817 includes a portion functioning as the gateelectrode of the transistor 111 and is connected to the conductive layer3812 through a wiring 3818. A conductive layer 3819 includes a portionfunctioning as the third scan line 120 and the gate electrode of thethird switching transistor 414. A conductive layer 3820 includes aportion functioning as the first electrode of the third switchingtransistor 414 and is connected to the pixel electrode 3844. Aconductive layer 3821 including a portion functioning as the secondelectrode of the third switching transistor 414 is connected to thepotential supply line 122 through a wiring 3823.

Note that among the above conductive layers, the portions functioning asthe gate electrode, the first electrode, and the second electrode of thefirst switching transistor 412 are portions formed so as to beoverlapped with a semiconductor layer 3833; the portions functioning asthe gate electrode, the first electrode, and the second electrode of thesecond switching transistor 413 are portions formed so as to beoverlapped with a semiconductor layer 3834; and the portions functioningas the gate electrode, the first electrode, and the second electrode ofthe third switching transistor 414 are portions formed so as to beoverlapped with a semiconductor layer 3835. In addition, the portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the transistor 111 are portions of conductive layers formedso as to be overlapped with a semiconductor layer 3836. The capacitor115 is formed in a portion where the conductive layer 3812 and the pixelelectrode 3844 overlap.

The pixel structure in FIG. 4 can also suppress variation in currentvalue caused by variation in threshold voltage of the transistor 111 byan operating method similar to FIG. 1. Thus, current in accordance withluminance data can be supplied to the light emitting element 116, andvariation in luminance can be suppressed. When the transistor 111 isoperated in the saturation region, variation in luminance caused bydeterioration of the light emitting element 116 can also be suppressed.

Further, a manufacturing process can be simplified because the pixel canbe formed using only n-channel transistors. In addition, anon-crystalline semiconductor such as an amorphous semiconductor or asemi-amorphous semiconductor (also referred to as a microcrystallinesemiconductor) can be used for a semiconductor layer of each transistorincluded in the pixel. For example, amorphous silicon (a-Si:H) can beused as the amorphous semiconductor. The manufacturing process canfurther be simplified by using these non-crystalline semiconductors.Accordingly, a reduction in manufacturing cost and an improvement inyield can be achieved.

Note that the first switching transistor 412, the second switchingtransistor 413, and the third switching transistor 414 are operated asmere switches. Therefore, the polarity (conductivity type) of thetransistors is not particularly limited. However, it is desirable to usea transistor having a polarity with lower off-current. As the transistorwith low off-current, a transistor provided with an LDD region, atransistor having a multi-gate structure, or the like can be used.Alternatively, the switch may be of CMOS type using both an n-channeltransistor and a p-channel transistor.

Next, a display device including the pixel of the present invention isexplained with reference to FIG. 6.

The display device includes a signal line driver circuit 611, a scanline driver circuit 612, and a pixel portion 613, and the pixel portion613 includes a plurality of signal lines S1 to Sm which is arrangedextending from the signal line driver circuit 611 in a column direction,a plurality of first scan lines G1_1 to Gn_1, second scan lines G1_2 toGn_2, third scan lines G1_3 to Gn_3, and power supply lines P1_1 to Pn_1which are arranged extending from the scan line driver circuit 612 in arow direction, and a plurality of pixels 614 which is arranged in matrixin accordance with the signal lines S1 to Sm. In addition, the pixelportion 613 includes a plurality of potential supply lines P1_2 to Pn_2parallel to the first scan lines G1_1 to Gn_. Further, each pixel 614 isconnected to a signal line Sj (one of the signal lines S1 to Sm), afirst scan line Gi_1 (one of the scan lines G1_1 to Gn_1), a second scanline Gi_2, a third scan line Gi_3, a power supply line Pi_1, and apotential supply line Pi_2.

Note that the signal line Sj, the first scan line Gi_1, the second scanline Gi_2, the third scan line Gi_3, the power supply line Pi_1, and thepotential supply line Pi_2 correspond to the signal line 117, the firstscan line 118, the second scan line 119, the third scan line 120, thepower supply line 121, and the potential supply line 122 in FIG. 1,respectively.

In accordance with signals outputted from the scan line driver circuit612, a row of pixels to be operated is selected, and at the same time,the operation shown in FIG. 2 is performed in each of the pixelsbelonging to the row. Note that in the data write period of FIG. 2, avideo signal outputted from the signal line driver circuit 611 iswritten into each pixel of the selected row. At this time, potentials inaccordance with luminance data of pixels are inputted to the signallines S1 to Sm.

As shown in FIG. 40, after finishing a data write period of for example,the i-th row, a signal is written in pixels belonging to the i+l-th row.Note that in order to show the data write period of each row, FIG. 40shows only the operation of the first switch 112 of FIG. 2 which canprecisely show the period. In addition, a pixel that has finished thedata write period in the i-th row proceeds to a light emitting periodand emits light in accordance with the signal written into the pixel.

Thus, initialization start time can be freely set in respective rowsunless data write periods overlap in the respective rows. In addition,since each pixel can emit light except in its address period, a ratio ofa light emitting period to one frame period (that is, a duty ratio) canbe significantly high and can be approximately 100%. Therefore, adisplay device with less luminance variation and a high duty ratio canbe provided.

In addition, since a threshold write period can be set to be long, athreshold voltage of a transistor can be written into a capacitor moreaccurately. Therefore, reliability as a display device is improved.

Note that the structure of the display device shown in FIG. 6 is oneexample, and the present invention is not limited to this. For example,the potential supply lines P1_2 to Pn_2 do not need to be arrangedparallel to the first scan lines G1_1 to Gn_1, and may be arrangedparallel to the signal lines S1 to Sm.

In addition, the variation in threshold voltage includes fluctuation inthreshold voltage of each transistor over time as well as a differencein threshold voltage among transistors of pixels. Further, thedifference in threshold voltage among transistors includes a differencein transistor characteristic at the time of manufacturing thetransistors. Note that the transistor here refers to a transistorfunctioning to supply current to a load such as a light emittingelement.

Embodiment Mode 2

In this embodiment mode, a pixel with a structure different fromEmbodiment Mode 1 is explained with reference to FIGS. 39A and 39B. Notethat a common reference numeral is used to denote a component similar toEmbodiment Mode 1, and detailed explanation of the same portion or aportion having a similar function is omitted.

The pixel shown in FIG. 39A includes a transistor 111, a first switch112, a second switch 113, a rectifier element 3914, a capacitor 115, anda light emitting element 116. Note that the pixel is connected to asignal line 117, a first scan line 118, a second scan line 119, a thirdscan line 3920, and a power supply line 121. The pixel shown in FIG. 39Ahas a structure in which the rectifier element 3914 is used as the thirdswitch 114 in FIG. 1, and a second electrode of the capacitor 115, afirst electrode of the transistor 111, and a pixel electrode of thelight emitting element 116 are connected to the third scan line 3920through the rectifier element 3914. In other words, the rectifierelement 3914 is connected so that current flows to the third scan line3920 from the first electrode of the transistor 111. It is needless tosay that transistors or the like may be used as the first switch 112 andthe second switch 113 as described in Embodiment Mode 1. In addition,for the rectifier element 3914, a Schottky-barrier diode 3951, a PINdiode 3952, or a PN diode 3953 or a diode-connected transistor 3954 or3955 shown in FIG. 39B can be used. Note that, as for the transistors3954 and 3955, the polarity of the transistors needs to be appropriatelyselected depending on a direction of current flow.

Current does not flow to the rectifier element 3914 when an H-levelsignal is inputted to the third scan line 3920, and current flows to therectifier element 3914 when an L-level signal is inputted. Thus, whenthe pixel in FIG. 39A is operated in a similar manner to that in FIG. 1,an L-level signal is inputted to the third scan line 3920 in theinitialization period, and an H-level signal is inputted in the otherperiods. Note that a potential of the L-level signal is considered to beV1-Vth-α-β (α: an arbitrary positive number) because it is required notonly that current flow to the rectifier element 3914 but also that apotential of the second electrode of the capacitor 115 be lowered toV1-Vth-α (α: an arbitrary positive number). Note that 0 denotes athreshold voltage of the rectifier element 3914 in a forward direction.

In consideration of the above matters, the pixel structure of FIGS. 39Aand 39B can also suppress variation in current value caused by variationin threshold voltage of the transistor 111 by operating the pixel in asimilar manner to FIG. 1. Thus, current in accordance with luminancedata can be supplied to the light emitting element 116, and variation inluminance can be suppressed. In addition, in the case of operating thetransistor 111 in the saturation region, variation in luminance causedby deterioration of the light emitting element 116 can also besuppressed. Further, the use of the rectifier element 3914 can reducethe number of wirings and improve an aperture ratio.

Furthermore, the pixel described in this embodiment mode can be appliedto the display device of FIG. 6. Similarly to Embodiment Mode 1,initialization start time can be freely set in respective rows unlessdata write periods in the respective rows overlap. In addition, sinceeach pixel can emit light except in its address period, a ratio of alight emitting period to one frame period (that is, a duty ratio) can besignificantly high and can be approximately 100%. Therefore, a displaydevice with less luminance variation and a high duty ratio can beprovided.

In addition, since a threshold write period can be set to be long, athreshold voltage of a transistor which controls a current value flowingto a light emitting element can be written into a capacitor moreaccurately. Therefore, reliability as a display device is improved.

This embodiment mode can be freely combined with a pixel structuredescribed in the other embodiment mode as well as FIG. 1 describedabove. In other words, the rectifier element 3914 can be applied to apixel described in the other embodiment mode.

Embodiment Mode 3

In this embodiment mode, pixels having structures different from thosein Embodiment Mode 1 are explained with reference to FIGS. 7 to 9. Notethat a common reference numeral is used to denote a component similar toEmbodiment Mode 1, and detailed explanation of the same portion or aportion having a similar function is omitted.

A pixel 700 shown in FIG. 7 includes a transistor 111, a first switch112, a second switch 113, a third switch 114, a capacitor 115, and alight emitting element 116. Note that the pixel 700 is connected to asignal line 117, a first scan line 718, a second scan line 119, a thirdscan line 120, a power supply line 121, and a first pixel line 718 of asubsequent row.

In the pixel of FIG. 1 described in Embodiment Mode 1, the firstelectrode of the transistor 111 is connected to the potential supplyline 122 through the third switch 114, while it can be connected to thefirst scan line 718 of the subsequent row in FIG. 7. This is because thepotential supply line 112 can be replaced with anything that can supplya predetermined potential to the first electrode of the transistor 11 lin the initialization period. Therefore, a supplying wiring does notalways need to be at a constant potential as long as the wiring cansupply a predetermined potential to the first electrode of thetransistor 111 in the initialization period. Thus, the first scan line718 of the subsequent row can be used in place of the potential supplyline. By sharing a wiring with a subsequent row as described above, thenumber of wirings can be reduced and an aperture ratio can be improved.

Note that the pixel structure shown in FIG. 7 can also suppressvariation in current value caused by variation in threshold voltage ofthe transistor 111 by operating the pixel in a similar manner toEmbodiment Mode 1. Thus, current in accordance with luminance data canbe supplied to the light emitting element 116, and variation inluminance can be suppressed. In addition, power consumption can bereduced because operation is performed with an opposite electrode fixedat a constant potential. Note that although the operation region of thetransistor 111 is not particularly limited, a more notable effect can beobtained in the saturation region. Further, when the transistor 111 isoperated in the saturation region, variation in current flowing to thetransistor 111 caused by deterioration of the light emitting element 116can be suppressed.

Note that a potential of a signal to turn off the first switch 112 whichis supplied from the first scan line 718 is V1-Vth-α (α: an arbitrarypositive number). Therefore, it is necessary to use the first switch 112which is turned off with the potential V1-Vth-α (α: an arbitrarypositive number). It is also necessary to perform operation so that theinitialization period of the row to which the pixel 700 belongs does notoverlap with a data write period of the row sharing a wiring.

Note that in the case of using an n-channel transistor for the thirdswitch 114, a potential to turn off the third switch 114 which issupplied from the third scan line 120 may be lower than the potentialV1-Vth-α that is the potential of the signal to turn off the firstswitch 112 which is supplied from the first scan line 718. In this case,a gate-source voltage when the transistor is turned off can be anegative value. Thus, current leakage when the third switch 114 isturned off can be reduced.

In addition, as shown in a pixel 800 of FIG. 8, a second scan line 819of a subsequent row may also be used as the potential supply line 122 ofFIG. 1. The pixel 800 can also perform similar operation to EmbodimentMode 1. Note that a potential of a signal to turn off the second switch113 which is supplied from the second scan line 819 is V1-Vth-α (α: anarbitrary positive number). Therefore, it is necessary to use the secondswitch 113 which is turned off with the potential V1-Vth-α (α: anarbitrary positive number). It is also necessary to perform operation sothat the initialization period of the row to which the pixel 800 belongsdoes not overlap with a threshold write period of the row sharing awiring.

Note that in the case of using an n-channel transistor for the thirdswitch 114, a potential of a signal to turn off the third switch 114which is supplied from the third scan line 120 may be lower than thepotential V1-Vth-α that is the potential of the signal to turn off thesecond switch 113 which is supplied from the second scan line 819. Inthis case, current leakage when the third switch 114 is turned off canbe reduced.

In addition, as shown in a pixel 900 of FIG. 9, the potential supplyline 122 of FIG. 1 may also be used as a third scan line 920 of apreceding row. The pixel 900 can also perform similar operation toEmbodiment Mode 1. Note that a potential of a signal to turn off a thirdswitch 114 which is supplied from the third scan line 920 is V1-Vth-α(α: an arbitrary positive number). Therefore, it is necessary to use thethird switch 114 which is turned off with the potential V1-Vth-α (α: anarbitrary positive number). It is also necessary to perform operation sothat the initialization period of the row to which the pixel 900 belongsdoes not overlap with an initialization period of the row sharing awiring. However, there is no particular problem when the initializationperiod is set to be shorter than the data write period.

Note that although this embodiment mode describes the case of using thepotential supply line 122 of FIG. 1 also as a scan line of a subsequentor preceding row, any other wiring can be used as long as it can supplythe potential V1-Vth-α (α: an arbitrary positive number) in theinitialization period.

Further, the pixels described in this embodiment mode can be applied tothe display device of FIG. 6. Note that initialization start time can befreely set in respective rows in the display device within the range inwhich there is limitation on operation of each of the pixels shown inFIGS. 7 to 9 and data write periods in the respective rows do notoverlap. In addition, since each pixel can emit light except in itsaddress period, a ratio of a light emitting period to one frame period(that is, a duty ratio) can be significantly high and can beapproximately 100%. Therefore, a display device with less luminancevariation and a high duty ratio can be provided.

In addition, since a threshold write period can be set to be long, athreshold voltage of a transistor which controls a current value flowingto a light emitting element can be written into a capacitor moreaccurately. Therefore, reliability as a display device is improved.

This embodiment mode can be freely combined with any of the pixelstructures described in Embodiment Modes 1 and 2 besides FIG. 1described above.

Embodiment Mode 4

In this embodiment mode, a pixel having a structure different fromEmbodiment Mode 1 is explained with reference to FIG. 10. Note that acommon reference numeral is used to denote a component similar toEmbodiment Mode 1, and detailed explanation of the same portion or aportion having a similar function is omitted.

A pixel shown in FIG. 10 includes a transistor 1011, a first switch 112,a second switch 113, a third switch 114, a capacitor 115, and a lightemitting element 116. Note that the pixel is connected to a signal line117, a first scan line 118, a second scan line 119, a third scan line120, a power supply line 121, and a potential supply line 122.

The transistor 1011 in this embodiment mode is a multi-gate transistorin which two transistors are connected in series, and is provided in thesame position as the transistor 111 in Embodiment Mode 1. Note that thenumber of transistors connected in series is not particularly limited.

By operating the pixel shown in FIG. 10 in a similar manner toEmbodiment Mode 1, variation in current value caused by variation inthreshold voltage of the transistor 1011 can be suppressed. Thus,current in accordance with luminance data can be supplied to the lightemitting element 116, and variation in luminance can be suppressed. Inaddition, power consumption can be reduced because operation isperformed with an opposite electrode fixed at a constant potential. Notethat although the operation region of the transistor 1011 is notparticularly limited, a more notable effect can be obtained in thesaturation region.

Further, when the transistor 1011 is operated in the saturation region,variation in current flowing to the transistor 1011 caused bydeterioration of the light emitting element 116 can be suppressed.

When channel widths of the two transistors connected in series are equalto each other, a channel length L of the transistor 1011 in thisembodiment mode is equal to the sum of channel lengths of the respectivetransistors. Thus, a current value which is closer to a constant valuecan be easily obtained in the saturation region regardless of adrain-source voltage Vds. In particular, the transistor 1011 iseffective when it is difficult to manufacture a transistor having a longchannel length L. Note that a connection portion of the two transistorsfunctions as a resistor.

Note that it is acceptable as long as the transistor 1011 functions tocontrol a current value supplied to the light emitting element 116, andthe kind of the transistor is not particularly limited. Therefore, athin film transistor (TFT) using a crystalline semiconductor film, athin film transistor using a non-single crystalline semiconductor filmtypified by an amorphous silicon film or a polycrystalline silicon film,a transistor formed using a semiconductor substrate or an SOI substrate,a MOS transistor, a junction transistor, or a bipolar transistor, atransistor using an organic semiconductor or a carbon nanotube, oranother transistor can be used.

In the pixel shown in FIG. 10, transistors can be used for the firstswitch 112, the second switch 113, and the third switch 114 similarly tothe pixel shown in FIG. 1.

Furthermore, the pixel described in this embodiment mode can be appliedto the display device of FIG. 6. Similarly to Embodiment Mode 1,initialization start time can be freely set in respective rows unlessdata write periods in the respective rows overlap. In addition, sinceeach pixel can emit light except in its address period, a ratio of alight emitting period to one frame period (that is, a duty ratio) can besignificantly high and can be approximately 100%. Therefore, a displaydevice with less luminance variation and a high duty ratio can beprovided.

In addition, since a threshold write period can be set to be long, athreshold voltage of a transistor which controls a current value flowingto a light emitting element can be written into a capacitor moreaccurately. Therefore, reliability as a display device is improved.

Note that the structure of the transistor 1011 is not limited to that inwhich transistors are connected in series, and may be that in whichtransistors are connected in parallel like a transistor 1111 shown inFIG. 11. The transistor 1111 can supply larger current to the lightemitting element 116. In addition, since transistor characteristics areaveraged by two transistors connected in parallel, originalcharacteristic variation of the transistors included in the transistor1111 can be reduced. When the variation is reduced, variation in currentvalue caused by variation in threshold voltage of the transistor can besuppressed more easily by the operation shown in FIG. 2.

This embodiment mode can also be applied to any of the pixel structuresdescribed in the other embodiment modes as well as FIG. 1 describedabove.

Embodiment Mode 5

In this embodiment mode, a pixel structure which averages deteriorationof transistors over time by periodically switching the transistors whichcontrol a current value supplied to a light emitting element in thepixel of the present invention is explained with reference to FIG. 12.

A pixel shown in FIG. 12 includes a first transistor 1201, a secondtransistor 1202, a first switch 1212, a second switch 1213, a thirdswitch 1214, a fourth switch 1203, a fifth switch 1204, a capacitor1215, and a light emitting element 1216. Note that the pixel isconnected to a signal line 1217, a first scan line 1218, a second scanline 1219, a third scan line 1220, a power supply line 1221, and apotential supply line 1222. In addition, although not shown in FIG. 12,the pixel is also connected to fourth and fifth scan lines which controlthe fourth switch 1203 and the fifth switch 1204 to be turned on andoff. In this embodiment mode, the first transistor 1201 and the secondtransistor 1202 are n-channel transistors, and each transistor is turnedon when a gate-source voltage (Vgs) exceeds a threshold voltage. Inaddition, a pixel electrode of the light emitting element 1216 is ananode, and an opposite electrode 1223 thereof is a cathode. Note that agate-source voltage of a transistor is referred to as Vgs and a voltageaccumulated in a capacitor is referred to as Vcs. A threshold voltage ofthe first transistor 1201 is referred to as Vth1 and that of the secondtransistor 1202 is referred to as Vth2. The power supply line 1221, thepotential supply line 1222, and the signal line 1217 are referred to asa first wiring, a second wiring, and a third wiring, respectively.

A first electrode of the first transistor 1201 is connected to the pixelelectrode of the light emitting element 1216 through the fourth switch1203; a second electrode thereof, to the power supply line 1221; and agate electrode thereof, to the power supply line 1221 through the secondswitch 1213. In addition, the gate electrode of the first transistor1201 is also connected to the signal line 1217 through the first switch1212, and the first electrode of the first transistor 1201 is alsoconnected to the potential supply line 1222 through the fourth switch1203 and the third switch 1214.

A first electrode of the second transistor 1202 is connected to thepixel electrode of the light emitting element 1216 through the fifthswitch 1204; a second electrode thereof, to the power supply line 1221;and a gate electrode thereof, to the power supply line 1221 through thesecond switch 1213. The gate electrode of the second transistor 1202 isalso connected to the signal line 1217 through the first switch 1212,and the first electrode of the second transistor 1202 is also connectedto the potential supply line 1222 through the fifth switch 1204 and thethird switch 1214. Note that the gate electrodes of the first transistor1201 and the second transistor 1202 are connected to each other, thesecond electrodes of the first transistor 1201 and the second transistor1202 are connected to each other; and the first electrodes of the firsttransistor 1201 and the second transistor 1202 are connected to eachother though the fourth switch 1203 and the fifth switch 1204.

Furthermore, the connected gate electrodes of the first transistor 1201and the second transistor 1202 are connected to the first electrode ofthe first transistor 1201 through the capacitor 1215 and the fourthswitch 1203 and also connected to the first electrode of the secondtransistor 1202 through the capacitor 1215 and the fifth switch 1204. Inother words, a first electrode of the capacitor 1215 is connected to thegate electrodes of the first transistor 1201 and the second transistor1202 and a second electrode of the capacitor 1215 is connected to thefirst electrodes of the first transistor 1201 and the second transistor1202 through the respective switches. Note that the capacitor 1215 maybe formed by sandwiching an insulating film between a wiring, asemiconductor layer, and an electrode or can be omitted by using gatecapacitances of the first transistor 1201 and the second transistor1202.

Note that the first switch 1212, the second switch 1213, and the thirdswitch 1214 are controlled to be turned on and off by inputting signalsto the first scan line 1218, the second scan line 1219, and the thirdscan line 1220, respectively. In FIG. 12, scan lines which control thefourth switch 1203 and the fifth switch 1204 to be turned on and off areomitted. [0129]A signal in accordance with a pixel gray scale levelwhich corresponds to a video signal, that is, a potential in accordancewith luminance data is inputted to the signal line 1217.

Next, the operation of the pixel shown in FIG. 12 is explained withreference to a timing chart of FIG. 13. Note that one frame periodcorresponding to a period for displaying an image for one screen in FIG.13 is divided into an initialization period, a threshold write period, adata write period, and a light emitting period.

Note that a potential V1 is supplied to the opposite electrode 1223 ofthe light emitting element 1216, and when a higher value between Vth1and Vth2 is referred to as Vth, a potential V1-Vth-α (α: an arbitrarypositive number) is supplied to the potential supply line 1222. Inaddition, the potential V1 and a potential V2 are supplied to the powersupply line 1221 in an address period and in the light emitting period,respectively. Note that V2>V1.

Here, the potential of the opposite electrode 1223 of the light emittingelement 1216 is equal to the potential of the power supply line 1221 inthe address period for the purpose of explaining operation. However,when the minimum potential difference which is necessary for the lightemitting element 1216 to emit light is referred to as V_(EL) it isacceptable as long as the potential of the opposite electrode 1223 ishigher than a potential V1-Vth-α-V_(EL). In addition, it is acceptableas long as the potential V2 of the power supply line 1221 in the lightemitting period is higher than the sum of the potential of the oppositeelectrode 1223 and the minimum potential difference (V_(EL)) which isnecessary for the light emitting element 1216 to emit light. However,since the potential of the opposite electrode 1223 is V1 here for thepurpose of explanation, it is acceptable as long as V2 is higher thanV1+V_(E).

First, in the initialization period as shown in (A) in FIG. 13, thefirst switch 1212 and the fifth switch 1204 are turned off and thesecond switch 1213, the third switch 1214, and the fourth switch 1203are turned on. At this time, the first electrode of the first transistor1201 serves as a source electrode, and a potential thereof is V1-Vth-α.On the other hand, a potential of the gate electrode of the firsttransistor 1201 is V1. Thus, a gate-source voltage Vgs of the firsttransistor 1201 is Vth+α and thus the first transistor 1201 is turnedon. Then, Vth+α is held by the capacitor 1215 positioned between thegate electrode and the first electrode of the first transistor 1201.

Next, in the threshold write period shown in (B) in FIG. 13, the thirdswitch 1214 is turned off. Therefore, the potential of the firstelectrode, i.e. the source electrode of the first transistor 1201 risesgradually and when it reaches V1-Vth1, the first transistor 1201 isturned off. Thus, a voltage held by the capacitor 1215 is Vth1.

Next, in the data write period shown in (C) in FIG. 13, the first switch1212 is turned on and a potential (V1+Vdata) in accordance withluminance data is inputted from the signal line 1217 after turning offthe second switch 1213. At this time, the voltage Vcs held by thecapacitor 1215 is Vth1+Vdata, and thus the first transistor 1201 isturned on. Note that when a potential Vdata≤0 is inputted, the firsttransistor 1201 can be turned off so that the light emitting element1216 does not emit light.

Next, in the light emitting period shown in (D) in FIG. 13, the firstswitch 1212 is turned off and the potential of the power supply line1221 is set to V2. At this time, the gate-source voltage Vgs of thefirst transistor 1201 is equal to Vth1+Vdata, and current in accordancewith this Vgs flows to the first transistor 1201 and the light emittingelement 1216, so that the light emitting element 1216 emits light.

According to such operation, the current flowing to the light emittingelement 1216 does not depend on the threshold voltage (Vth1) of thefirst transistor 1201 regardless of whether the operation region of thefirst transistor 1201 is either the saturation region or the linearregion.

Furthermore, in an initialization period of a subsequent one frameperiod shown in (E) in FIG. 13, the fourth switch 1203 is turned off andthe second switch 1213, the third switch 1214, and the fifth switch 1204are turned on. At this time, the first electrode of the secondtransistor 1202 serves as a source electrode, and a potential thereof isV1-Vth-α. On the other hand, a potential of the gate electrode of thesecond transistor 1202 is V1. Thus, a gate-source voltage Vgs of thesecond transistor 1202 is Vth+a, so that the second transistor 1202 isturned on. Then, Vth+α is held by the capacitor 1215 positioned betweenthe gate electrode and the first electrode of the second transistor1202.

Next, in the threshold write period shown in (F) in FIG. 13, the thirdswitch 1214 is turned off. Therefore, the potential of the firstelectrode, i.e. the source electrode of the second transistor 1202 risesgradually and when it reaches V1-Vth2, the second transistor 1202 isturned off. Thus, a voltage held by the capacitor 1215 is Vth2.

Next, in the data write period shown in (G) in FIG. 13, the first switch1212 is turned on and a potential (V1+Vdata) in accordance withluminance data is inputted from the signal line 1217 after turning offthe second switch 1213. At this time, the voltage Vcs held by thecapacitor 1215 is Vth2+Vdata, and thus the second transistor 1202 isturned on.

Next, in the light emitting period shown in (H) in FIG. 13, the firstswitch 1212 is turned off and the potential of the power supply line1221 is set to V2. At this time, the gate-source voltage Vgs of thesecond transistor 1202 is equal to Vth2+Vdata, and current in accordancewith this Vgs flows to the second transistor 1202 and the light emittingelement 1216, so that the light emitting element 1216 emits light.

The current flowing to the light emitting element 1216 does not dependon the threshold voltage (Vth2) regardless of whether the operationregion of the second transistor 1202 is either the saturation region orthe linear region.

Therefore, by controlling current supplied to the light emitting elementusing either the first transistor 1201 or the second transistor 1202,variation in current value caused by variation in threshold voltage ofthe transistor can be suppressed and a current value in accordance withluminance data can be supplied to the light emitting element 1216. Notethat by reducing a load on each transistor by switching between thefirst transistor 1201 and the second transistor 1202, changes over timein threshold voltage of the transistors can be reduced.

Accordingly, variation in luminance caused by variation in thresholdvoltages of the first transistor 1201 and the second transistor 1202 canbe suppressed. In addition, since the potential of the oppositeelectrode is fixed, power consumption can be reduced.

Further, in the case of operating the first transistor 1201 and thesecond transistor 1202 in the saturation region, variation in currentflowing to each transistor due to deterioration of the light emittingelement 1216 can also be suppressed.

Note that in the case of operating the first transistor 1201 and thesecond transistor 1202 in the saturation region, the channel lengths Lof these transistors are preferably long.

In addition, since a reverse bias voltage is applied to the lightemitting element 1216 in the initialization period, a shorted portion inthe light emitting element can be insulated and deterioration of thelight emitting element can be suppressed. Thus, the lifetime of thelight emitting element can be extended.

Note that since the variation in current value caused by variation inthreshold voltage of the transistors can be suppressed, supplydestination of the current controlled by the transistors is notparticularly limited. Therefore, an EL element (an organic EL element,an inorganic EL element, or an EL element containing an organic materialand an inorganic material), an electron emitting element, a liquidcrystal element, electronic ink, or the like can be used as the lightemitting element 1216 shown in FIG. 12.

In addition, it is acceptable as long as the first transistor 1201 andthe second transistor 1202 function to control a current value suppliedto the light emitting element 1216, and the kind of the transistors isnot particularly limited. Therefore, a thin film transistor (TFT) usinga crystalline semiconductor film, a thin film transistor using anon-single crystalline semiconductor film typified by an amorphoussilicon film or a polycrystalline silicon film, a transistor formedusing a semiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, a transistor using an organicsemiconductor or a carbon nanotube, or another transistor can be used.

The first switch 1212 selects timing to input a signal in accordancewith a pixel gray scale level to the capacitor. The second switch 1213selects timing to apply a predetermined potential to the gate electrodeof the first transistor 1201 or the second transistor 1202. The thirdswitch 1214 selects timing to apply a predetermined potential forinitializing a potential written in the capacitor 1215. Therefore, thefirst switch 1212, the second switch 1213, and the third switch 1214 arenot particularly limited as long as they have the above functions. Forexample, each of the switches may be a transistor, a diode, or a logiccircuit that is a combination thereof. Note that the first to thirdswitches are not particularly necessary if the signal or potential canbe applied to the pixel at the above timing. Further, the fourth switch1203 and the fifth switch 1204 are also not particularly limited, eachof which may be, for example, a transistor, a diode, or a logic circuitthat is a combination thereof.

In the case of using n-channel transistors for the first switch 1212,the second switch 1213, the third switch 1214, the fourth switch 1203,and the fifth switch 1204, a manufacturing process can be simplifiedbecause the pixel can be formed using only an n-channel transistors. Inaddition, a non-crystalline semiconductor such as an amorphoussemiconductor or a semi-amorphous semiconductor (also referred to as amicrocrystalline semiconductor) can be used for a semiconductor layer ofeach transistor included in the pixel. For example, amorphous silicon(a-Si:H) can be used as the amorphous semiconductor. The manufacturingprocess can further be simplified by using these non-crystallinesemiconductors. Accordingly, a reduction in manufacturing cost and animprovement in yield can be achieved.

Note that in the case of using transistors for the first switch 1212,the second switch 1213, the third switch 1214, the fourth switch 1203,and the fifth switch 1204, the polarity (conductivity type) of thetransistors is not particularly limited. However, it is desirable to usea transistor having a polarity with lower off-current.

In addition, the first transistor 1201 and the fourth switch 1203, andthe second transistor 1202 and the fifth switch 1204 are interchangeableas shown in FIG. 41. In other words, the first electrodes of the firsttransistor 1201 and the second transistor 1202 are connected to the gateelectrodes of the first transistor 1201 and the second transistor 1202through the capacitor 1215. The second electrode of the first transistor1201 is connected to the power supply line 1221 through the fourthswitch 1203, and the second electrode of the second transistor 1202 isconnected to the power supply line 1221 through the fifth switch 1204.

FIGS. 12 and 41 show the case where the number of elements arranged inparallel is two using a transistor and a switch as a set, that is, usingthe first transistor 1201 and the fourth switch 1203 as a set, and thesecond transistor 1202 and the fifth switch 1204 as a set. However, thenumber of elements arranged in parallel is not particularly limited.

By applying the pixel described in this embodiment mode to the displaydevice of FIG. 6, initialization start time can be freely set inrespective rows similarly to Embodiment Mode 1 unless data write periodsin the respective rows overlap. In addition, since each pixel can emitlight except in its address period, a ratio of a light emitting periodto one frame period (that is, a duty ratio) can be significantly highand can be approximately 100%. Therefore, a display device with lessluminance variation and a high duty ratio can be provided.

In addition, since a threshold write period can be set to be long, athreshold voltage of a transistor which controls a current value flowingto a light emitting element can be written into a capacitor moreaccurately. Thus, reliability as a display device is improved.

Note that the potential supply line 1222 can also be used as a wiring ofanother row similarly to Embodiment Mode 3. In addition, similarly toEmbodiment Mode 4, a multi-gate transistor in which transistors areconnected in series, or transistors arranged in parallel may be used foreach of the first transistor 1201 and the second transistor 1202.Moreover, this embodiment mode can be applied to any of the pixelstructures described in Embodiment Modes 1 to 4.

Embodiment Mode 6

In this embodiment mode, a pixel having a structure different fromEmbodiment Mode 1 is described. A common reference numeral is used todenote a component similar to Embodiment Mode 1, and detailedexplanation of the same portion or a portion having a similar functionis omitted. Note that such portions are operated in a similar manner toEmbodiment Mode 1.

In this embodiment mode, a pixel structure which forcibly preventscurrent from flowing to a light emitting element 116 is explained. Inother words, this embodiment mode aims to obtain a display device inwhich an afterimage is hardly seen and moving image characteristics areexcellent by forcibly putting a light emitting element into a non-lightemitting state.

One of such pixel structures is shown in FIG. 29. A pixel shown in FIG.29 includes a fourth switch 2901 in addition to the transistor 111, thefirst switch 112, the second switch 113, the third switch 114, thecapacitor 115, and the light emitting element 116 which are included inthe pixel of FIG. 1. The pixel is connected to a fourth scan line 2902in addition to the signal line 117, the first scan line 118, the secondscan line 119, the third scan line 120, the power supply line 121, andthe potential supply line 122.

In FIG. 29, the fourth switch 2901 is connected in parallel with thecapacitor 115. Therefore, the gate electrode and the first electrode ofthe transistor 111 are short-circuited when the fourth switch 2901 isturned on. Then, since a gate-source voltage of the transistor 111 heldby the capacitor 115 can be 0 V, the transistor 111 is turned off andthe light emitting element 116 can be put into a non-light emittingstate. Note that the fourth switch 2901 can be controlled to be turnedon and off by scanning pixels from row to row with a signal inputted tothe fourth scan line 2902.

According to such operation, a signal written in the pixel is erased.Thus, an erase period in which the light emitting element is forciblyput in a non-light emitting state is provided until a subsequentinitialization period. In other words, black display is inserted.Accordingly, an afterimage becomes less perceptible and moving imagecharacteristics can be improved.

Meanwhile, as a driving method of a display device for expressing a grayscale, there are an analog gray scale method and a digital gray scalemethod. The analog gray scale method includes a method to control theemission intensity of a light emitting element in an analog manner and amethod to control the emission time of a light emitting element in ananalog manner. Between the two, the method to control the emissionintensity of a light emitting element in an analog manner is often used.On the other hand, in the digital gray scale method, a light emittingelement is turned on/off by control in a digital manner to express agray scale. The digital gray scale method has the advantage of highresistance to noise because processing can be performed using a digitalsignal. However, there are only two states, that is, a light emittingstate and a non-light emitting state, so that only two gray scale levelscan be expressed. Therefore, multiple level gray scale display isattempted by using another method in combination. As a technique formultiple level gray scale display, there are an area gray scale methodin which light emission area of a pixel is weighted and selected toperform gray scale display and a time gray scale method in which lightemission time is weighted and selected to perform gray scale display.

In the case of combining the digital gray scale method and the time grayscale method, one frame period is divided into a plurality of subframeperiods (SFn) as shown in FIG. 42. Each subframe period includes anaddress period (Ta) including an initialization period, a thresholdwrite period, and a data write period and a light emitting period (Ts).Note that subframe periods, the number of which corresponds to thenumber of display bits n, are provided in one frame period. In addition,a ratio of lengths of light emitting periods in respective subframeperiods is set to satisfy 2^((n-1)):2^((n-2)): . . . :2:1; lightemission or non-light emission of a light emitting element is selectedin each light emitting period; and a gray scale is expressed using adifference in total time in one frame period for which the lightemitting element emits light. When the total time of light emission inone frame period is long, luminance is high, and when short, luminanceis low. Note that FIG. 42 shows an example of a 4-bit gray scale, inwhich one frame period is divided into four subframe periods and 2⁴=16gray scale levels can be expressed by a combination of light emittingperiods. Note that a gray scale can also be expressed when a ratio oflengths of light emitting periods is not a power-of-two ratio. Further,a subframe period may further be divided.

Note that in the case of attempting multiple level gray scale display byusing the time gray scale method as described above, the length of alight emitting period of a lower-order bit is short. Therefore, whendata write operation is started immediately upon termination of a lightemitting period of a preceding subframe period, it overlaps with datawrite operation of the preceding subframe period, and thus normaloperation cannot be performed. Therefore, by providing an erase periodas described above in a subframe period, light emission shorter thandata write periods necessary for all rows can be expressed. In otherwords, a light emitting period can be freely set.

The present invention is effective particularly in the display analoggray scale method. Furthermore, it is effective to provide an eraseperiod because a light emitting period can be freely set also in amethod combining the digital gray scale method and the time gray scalemethod.

The erase period may be provided by providing another switch in the pathof current flow from the power supply line 121 to the pixel electrode ofthe light emitting element 116 through the transistor 111 and turningoff the switch by scanning pixels row by row.

One of such structures is shown in FIG. 30. In the structure of FIG. 30,a fourth switch 3001 is connected between the second electrode of thetransistor 111 and the power supply line 121, in addition to the pixelstructure of FIG. 1. The fourth switch 3001 is controlled to be turnedon and off by a signal inputted to a fourth scan line 3002.

When a connection point of the first electrode of the transistor 111 andthe pixel electrode of the light emitting element 116 is referred to asa node 3003, a fourth switch 3701 may be connected between the node 3003and the first electrode of the transistor 111 as shown in FIG. 37. Thefourth switch 3701 is controlled to be turned on and off by a signalinputted to the fourth scan line 3702.

Thus, an erase period can be provided by turning off the fourth switch.In addition, when operating the pixels shown in FIGS. 30 and 37similarly to Embodiment Mode 1, power consumption can also be reduced byturning off the fourth switch in an initialization period.

Note that the erase period can be provided by connecting a fourth switch4301 between the node 3003 and the pixel electrode of the light emittingelement 116 as shown in FIG. 43 as well as FIGS. 30 and 37.Alternatively, the erase period can be provided by connecting a fourthswitch 4401 between a connection point of the second electrode of thetransistor 111 and the second switch 113, and the power supply line 121as shown in FIG. 44.

Further, the erase period may be forcibly provided by inputting apotential to the gate electrode of the transistor 111.

One of such structures is shown in FIG. 31. The structure of FIG. 31includes a rectifier element 3101 in addition to the pixel structure ofFIG. 1, and the gate electrode of the transistor 111 and a fourth scanline 3102 are connected to each other through the rectifier element3101. Note that when the transistor 111 is an n-channel transistor, therectifier element 3101 is connected so that current flows from the gateelectrode of the transistor 111 to the fourth scan line 3102. As to thefourth scan line 3102, an L-level signal is inputted only when forciblyturning off the transistor 111, and otherwise, an H-level signal isinputted. Then, current does not flow to the rectifier element 3101 whenthe fourth scan line is at an H level, and current flows to the fourthscan line 3102 from the transistor 111 when at an L level. By causingcurrent to flow to the fourth scan line 3102 as described above, avoltage held by the capacitor 115 is lowered to the threshold voltage(Vth) of the transistor 111 or less, and the transistor 111 is forciblyturned off. Note that an L-level potential needs to be determined sothat the potential of the gate electrode of the transistor 111 does notbecome equal to or lower than a potential which is higher than theL-level potential by a forward threshold voltage of the rectifierelement 3101. In addition, in the case of using a switch which is turnedoff with the L-level potential for each of the first switch 112 and thesecond switch 113, the fourth scan line 3102 may be substituted with thefirst scan line 118 or the second scan line 119.

Note that a pixel structure is not particularly limited to the abovestructure because an afterimage can be made less perceptible byinserting black display as long as the pixel structure includes a meansto forcibly put a light emitting element into a non-light emittingstate.

Note that the Schottky-barrier diode, the PIN diode, the PN diode, thediode-connected transistor, or the like shown in FIG. 39B can be usedfor the rectifier element 3101.

Note that the switch for providing the erase period described in thisembodiment mode can be applied to the pixel structure described in anyof the other embodiment modes as well as that of FIG. 1 described above.

Without providing such a switch, the initialization period can alsoserve as the erase period by setting the initialization period to belong. Thus, moving image characteristics can be improved by setting thelength of a period in which black display is desired to be performed inorder to make an afterimage less perceptible to be equal to that of theinitialization period when operating any of the pixels described inEmbodiment Modes 1 to 5. Further, black display may be inserted byequalizing the potential of the power supply line 121 to the potentialof the opposite electrode 123 in the light emitting period.

Note that when the transistor 111 is turned on in the data write periodin the pixel structure shown in FIG. 30, current flow to the transistor111 can be blocked by turning off the fourth switch 3001. Thus, sincevariation in potential of the second electrode of the capacitor 115which is connected to the source electrode of the transistor 111 can besuppressed, a voltage Vth+Vdata can be held by the capacitor 115 moreaccurately. Consequently, more accurate current in accordance withluminance data can be supplied to the light emitting element 116.

In addition, since the pixel structure shown in FIG. 37 can alsosuppress variation in potential of the second electrode of the capacitor115 by turning off the fourth switch 3701 in the data write period alsoin, a voltage Vth+Vdata can be held by the capacitor 115 moreaccurately. Thus, more accurate current in accordance with luminancedata can be supplied to the light emitting element 116.

Note that the pixel described in this embodiment mode can be applied tothe display device described in Embodiment Mode 1. Accordingly, adisplay device with less luminance variation and excellent moving imagecharacteristics can be obtained.

Embodiment Mode 7

In this embodiment mode, the case of applying a p-channel transistor toa transistor which controls a current value supplied to a light emittingelement is explained with reference to FIG. 14.

A pixel shown in FIG. 14 includes a transistor 1411, a first switch1412, a second switch 1413, a third switch 1414, a capacitor 1415, and alight emitting element 1416. Note that the pixel is connected to asignal line 1417, a first scan line 1418, a second scan line 1419, athird scan line 1420, a power supply line 1421, and a potential supplyline 1422. In this embodiment mode, the transistor 1411 is a p-channeltransistor, and is turned on when the absolute value of its gate-sourcevoltage (|Vgs|) exceeds that of a threshold voltage (|Vth|) (when Vgs isbelow Vth). In addition, a pixel electrode of the light emitting element1416 is a cathode and an opposite electrode 1423 thereof is an anode.Note that the absolute value of a gate-source voltage of the transistoris denoted by |Vgs|, the absolute value of a threshold voltage isdenoted by |Vth|, and the power supply line 1421, the potential supplyline 1422, and the signal line 1417 are also referred to as a firstwiring, a second wiring, and a third wiring, respectively.

A first electrode (one of either a source electrode or a drainelectrode) of the transistor 1411 is connected to the pixel electrode ofthe light emitting element 1416; a second electrode (the other of eitherthe source electrode or the drain electrode) of the transistor 1411, tothe power supply line 1421; and a gate electrode of the secondtransistor 1411, to the power supply line 1421 through the second switch1413. In addition, the gate electrode of the transistor 1411 is alsoconnected to the signal line 1417 through the first switch 1412, and thefirst electrode thereof is also connected to the potential supply line1422 through the third switch 1414.

Further, the capacitor 1415 is connected between the gate electrode andthe first electrode of the transistor 1411. In other words, a firstelectrode of the capacitor 1415 is connected to the gate electrode ofthe transistor 1411 and a second electrode thereof is connected to thefirst electrode of the transistor 1411. Note that the capacitor 1415 maybe formed by sandwiching an insulating film between a wiring, asemiconductor layer, and an electrode or can be omitted by using a gatecapacitance of the transistor 1411.

Note that the first switch 1412, the second switch 1413, and the thirdswitch 1414 are controlled to be turned on and off by inputting signalsto the first scan line 1418, the second scan line 1419, and the thirdscan line 1420, respectively.

A signal in accordance with a pixel gray scale level which correspondsto a video signal, that is, a potential in accordance with luminancedata is inputted to the signal line 1417.

Next, the operation of the pixel shown in FIG. 14 is explained withreference to a timing chart of FIG. 15 and FIGS. 16A to 16D. Note thatone frame period corresponding to a period for displaying an image forone screen in FIG. 14 is divided into an initialization period, athreshold write period, a data write period, and a light emittingperiod. The initialization period, the threshold write period, and thedata write period are collectively referred to as an address period. Thelength of one frame period is not particularly limited, but ispreferably 1/60 second or less so that an image viewer does not perceiveflicker.

Note that a potential V1 is inputted to the opposite electrode 1423 ofthe light emitting element 1416 and a potential V1+|Vth|+α (α: anarbitrary positive number) is inputted to the potential supply line1422. In addition, to the power supply line 1421, V1 is inputted in theaddress period and a potential V2 is inputted in the light emittingperiod. Note that V2<V1.

Here, the potential of the opposite electrode 1423 of the light emittingelement 1416 is equal to the potential of the power supply line 1421 inthe address period for the purpose of explaining operation. However,when the minimum potential difference which is necessary for the lightemitting element 1416 to emit light is referred to as V_(EL), it isacceptable as long as the potential of the opposite electrode 1423 isequal to or greater than V1 and less than a potential V1+|Vth|+α+V_(EL).In addition, it is acceptable as long as the potential V2 of the powersupply line 1421 in the light emitting period is lower than a valueobtained by subtracting the minimum potential difference (V_(EL)) whichis necessary for the light emitting element 1416 to emit light from thepotential of the opposite electrode 1423. However, since the potentialof the opposite electrode 1423 is V1 here for the purpose ofexplanation, it is acceptable as long as V2 is less than V1-V_(EL).

First, the first switch 1412 is turned off and the second switch 1413and the third switch 1414 are turned on in the initialization period asshown in (A) in FIG. 15 and FIG. 16A. At this time, the first electrodeof the transistor 1411 serves as a source electrode, and a potentialthereof is equal to that of the potential supply line 1422, that is,V1+|Vth|+α. On the other hand, a potential of the gate electrode of thetransistor 1411 is V1. Thus, the absolute value of the gate-sourcevoltage |Vgs| of the transistor 1411 is |Vth|+α, and thus the transistor1411 is turned on. Then, |Vth|+α is held by the capacitor 1415 providedbetween the gate electrode and the first electrode of the transistor1411.

Next, the third switch 1414 is turned off in the threshold write periodshown in (B) in FIG. 15 and FIG. 16B. Therefore, the potential of thefirst electrode, i.e. the source electrode of the transistor 1411 dropsgradually and when it reaches V1+|Vth|, the transistor 1411 is turnedoff. Thus, a voltage held by the capacitor 1415 is |Vth|.

In the subsequent data write period shown in (C) in FIG. 15 and FIG.16C, the first switch 1412 is turned on and a potential (V1−Vdata) inaccordance with luminance data is inputted from the signal line 1417after turning off the second switch 1413. At this time, the voltage Vcsheld by the capacitor 1415 can be represented by Formula (5) wherecapacitances of the capacitor 1415 and the light emitting element 1416are referred to as C1 and C2, respectively.

$\begin{matrix}{{Vcs} = {{{- {{Vth}}} - {{Vdata} \times \frac{C\; 2}{{C\; 1} + {C\; 2}}}}}} & (5)\end{matrix}$

Note that C2>>C1 because the light emitting element 1416 is thinner andhas a larger electrode area than the capacitor 1415. Thus, fromC2/(C1+C2)−1, the voltage Vcs held by the capacitor 1415 is representedby Formula (6), and the transistor 1411 is turned on.

Vcs=|−|Vth|−Vdata|  (6)

Next, in the light emitting period shown in (D) in FIG. 15 and FIG. 16D,the first switch 1412 is turned off and the potential of the powersupply line 1421 is set to V2. At this time, the gate-source voltage Vgsof the transistor 1411 is equal to −Vdata−|Vth|, and current inaccordance with this Vgs flows to the transistor 1411 and the lightemitting element 1416, so that the light emitting element 1416 emitslight.

Note that a current I flowing to the light emitting element isrepresented by Formula (7) when the transistor 1411 is operated in thesaturation region.

$\begin{matrix}\begin{matrix}{I = {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{Vgs} - {Vth}} \right)}^{2}}} \\{= {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {{- {Vdata}} - {{Vth}} - {Vth}} \right)}^{2}}}\end{matrix} & (7)\end{matrix}$

Since the transistor 1411 is a p-channel transistor, Vth is less than 0.Thus, Formula (7) can be transformed into Formula (8).

$\begin{matrix}{I = {\frac{1}{2}\left( \frac{W}{L} \right)\mu \; {{Cox}\left( {- {Vdata}} \right)}^{2}}} & (8)\end{matrix}$

In addition, the current I flowing to the light emitting element isrepresented by Formula (9) when the transistor 1411 is operated in thelinear region.

$\begin{matrix}\begin{matrix}{I = {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{Vgs} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} \\{= {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {{- {Vdata}} - {{Vth}} - {Vth}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}}\end{matrix} & (9)\end{matrix}$

From Vth<0, Formula (9) can be transformed into Formula (10).

$\begin{matrix}{I = {\left( \frac{W}{L} \right)\mu \; {{Cox}\left\lbrack {{\left( {- {Vdata}} \right){Vds}} - {\frac{1}{2}{Vds}^{2}}} \right\rbrack}}} & (10)\end{matrix}$

Here, W denotes a channel width of the transistor 1411; L, a channellength; p, a mobility- and Cox, a storage capacitance.

According to Formulas (8) and (10), the current flowing to the lightemitting element 1416 does not depend on the threshold voltage (Vth) ofthe transistor 1411 regardless of whether the operation region of thetransistor 1411 is either the saturation region or the linear region.Therefore, variation in current value caused by variation in thresholdvoltage of the transistor 1411 can be suppressed and a current value inaccordance with luminance data can be supplied to the light emittingelement 1416.

Accordingly, variation in luminance caused by variation in thresholdvoltage of the transistor 1411 can be suppressed. In addition, powerconsumption can be reduced because operation is performed with theopposite electrode fixed at a constant potential.

Furthermore, when the transistor 1411 is operated in the saturationregion, variation in luminance due to deterioration of the lightemitting element 1416 can also be suppressed. When the light emittingelement 1416 is deteriorated, V_(EL) of the light emitting element 1416increases and the potential of the first electrode, that is, the sourceelectrode of the transistor 1411 decreases. At this time, the sourceelectrode of the transistor 1411 is connected to the second electrode ofthe capacitor 1415; the gate electrode of the transistor 1411 isconnected to the first electrode of the capacitor 1415; and the gateelectrode side is in a floating state. Therefore, in accordance with thedecrease in the source potential, the gate potential of the transistor1411 also decreases by the same amount as the decrease in the sourcepotential. Thus, Vgs of the transistor 1411 does not change. Therefore,the current flowing to the transistor 1411 and the light emittingelement 1416 is not affected even if the light emitting element isdeteriorated. Note that it is found also in Formula (8) that the currentI flowing to the light emitting element does not depend on the sourcepotential and a drain potential.

Therefore, when the transistor 1411 is operated in the saturationregion, variation in the luminance caused by variation in the thresholdvoltage of the transistor 1411 and deterioration of the light emittingelement 1416 can be suppressed.

Note that in the case of operating the transistor 1411 in the saturationregion, a channel length L of the transistor 1411 is preferably long tosuppress an increase in current amount by avalanche breakdown andchannel length modulation.

In addition, since a reverse bias voltage is applied to the lightemitting element 1416 in the initialization period, a shorted portion inthe light emitting element can be insulated and deterioration of thelight emitting element can be suppressed. Thus, the lifetime of thelight emitting element can be extended.

Note that the light emitting element 1416 shown in FIG. 14 is notparticularly limited, and an EL element (an organic EL element, aninorganic EL element, or an EL element containing an organic materialand an inorganic material), an electron emitting element, a liquidcrystal element, electronic ink, or the like can be used.

In addition, it is acceptable as long as the transistor 1411 functionsto control a current value supplied to the light emitting element 1416,and the kind of the transistor is not particularly limited. Therefore, athin film transistor (TFT) using a crystalline semiconductor film, athin film transistor using a non-single crystalline semiconductor filmtypified by an amorphous silicon film or a polycrystalline silicon film,a transistor formed using a semiconductor substrate or an SOI substrate,a MOS transistor, a junction transistor, or a bipolar transistor, atransistor using an organic semiconductor or a carbon nanotube, oranother transistor can be used.

The first switch 1412 selects timing to input a signal in accordancewith a pixel gray scale level to the capacitor. The second switch 1413selects timing to apply a predetermined potential to the gate electrodeof the transistor 1411. The third switch 1414 selects timing to apply apredetermined potential for initializing a potential written in thecapacitor 1415. Therefore, the first switch 1412, the second switch1413, and the third switch 1414 are not particularly limited as long asthey have the above functions. Each of the switches may be a transistor,a diode, or a logic circuit that is a combination thereof.

Note that in the case of using a transistor, a polarity (conductivitytype) thereof is not particularly limited. However, it is desirable touse a transistor having a polarity with lower off-current. As an exampleof the transistor having a polarity with lower off-current, a transistorprovided with an LDD region, a transistor having a multi-gate structure,or the like can be given. The switch may be of CMOS type using both ann-channel transistor and a p-channel transistor.

For example, in the case of applying p-channel transistors to the firstswitch 1412, the second switch 1413, and the third switch 1414, anL-level signal is inputted to a scan line which controls on/off of eachswitch when the switch is desired to be turned on, or an H-level signalis inputted when the switch is desired to be turned off.

In this case, a manufacturing process can be simplified because thepixel can be formed using only p-channel transistors.

Furthermore, the pixel described in this embodiment mode can be appliedto the display device of FIG. 6. Similarly to Embodiment Mode 1,initialization start time can be freely set in respective rows unlessdata write periods in the respective rows overlap. In addition, sinceeach pixel can emit light except in its address period, a ratio of alight emitting period to one frame period (that is, a duty ratio) can besignificantly high and can be approximately 100%. Therefore, a displaydevice with less luminance variation and a high duty ratio can beprovided.

In addition, since a threshold write period can be set to be long, athreshold voltage of a transistor which controls a current value flowingto a light emitting element can be written into a capacitor moreaccurately. Therefore, reliability as a display device is improved.

Note that the potential supply line 1422 can also be used as a wiring ofanother row similarly to Embodiment Mode 3. In addition, any of thestructures of the transistors described in Embodiment Modes 4 and 5 canbe applied to the transistor 1411. Moreover, the structure and operationdescribed in Embodiment Mode 6 can also be applied. In addition, thetransistor 1411 can be applied to the pixel structures described inEmbodiment Modes 1 to 6.

Note that in the case of using a rectifier element to provide an eraseperiod, the direction of current flowing to the rectifier element needsto be varied depending on the polarity of a transistor which controlscurrent flowing to a light emitting element. This is explained withreference to FIG. 32.

When the transistor 1411 is a p-channel transistor, a rectifier element3201 is connected so that current flows to the gate electrode of thetransistor 1411 from a fourth scan line 3202. As to the fourth scan line3202, an H-level signal is inputted only when forcibly turning off thetransistor 1411, and otherwise, an L-level signal is inputted. Then,current does not flow to the rectifier element 3201 when the fourth scanline 3202 is at an L level, and current flows to the fourth scan line3202 from the transistor 1411 when at an H level. By causing current toflow to the fourth scan line 3202 as described above, a potential heldby the capacitor 1415 is lowered to the absolute value of the thresholdvoltage (|Vth|) of the transistor 1411 or less, and thus the transistor1411 is forcibly turned off. Note that an H-level potential needs to bedetermined so that the potential of the gate electrode of the transistor1411 does not become equal to or higher than a potential which is lowerthan the H-level potential by a forward threshold voltage of therectifier element 3201. According to such operation, black display isinserted; an afterimage becomes less perceptible; and moving imagecharacteristics can be improved.

Embodiment Mode 8

In this embodiment mode, one mode of a fragmentary sectional view of apixel of the present invention is explained with reference to FIG. 17.Note that a transistor shown in the fragmentary sectional view in thisembodiment mode is a transistor functioning to control a current valuesupplied to a light emitting element.

First, a base film 1712 is formed over a substrate 1711 having aninsulating surface. As the substrate 1711 having an insulating surface,an insulating substrate such as a glass substrate, a quartz substrate, aplastic substrate (polyimide, acrylic, polyethylene terephthalate,polycarbonate, polyarylate, polyethersulfone, or the like), or a ceramicsubstrate; or a metal substrate (tantalum, tungsten, molybdenum, or thelike), a semiconductor substrate, or the like on the surface of which aninsulating film is formed, can be used. Note that it is necessary to usea substrate which can withstand at least heat generated during aprocess.

The base film 1712 is formed using a single layer or a plurality oflayers (two or more layers) of an insulating film such as a siliconoxide film, a silicon nitride film, or a silicon oxynitride(SiO_(x)N_(y)) film. Note that the base film 1712 may be formed using asputtering method, a CVD method, or the like. Although the base film1712 is a single layer in this embodiment mode, it goes without sayingthat it may be a plurality of layers (two or more layers).

Next, a transistor 1713 is formed over the base film 1712. Thetransistor 1713 includes at least a semiconductor layer 1714, a gateinsulating film 1715 formed over the semiconductor layer 1714, and agate electrode 1716 formed over the semiconductor layer 1714 with thegate insulating film 1715 interposed therebetween. The semiconductorlayer 1714 has a source region and a drain region.

The semiconductor layer 1714 can be formed using a film having anon-crystalline state (i.e. a non-crystalline semiconductor film)selected from an amorphous semiconductor containing silicon, silicongermanium (SiGe), or the like as well as amorphous silicon (a-Si:H) asits main component, a semi-amorphous semiconductor in which an amorphousstate and a crystalline state are mixed, a microcrystallinesemiconductor in which crystal grains of 0.5 nm to 20 nm can be observedwithin an amorphous semiconductor, or a crystalline semiconductor filmof polysilicon (p-Si:H) or the like. Note that the microcrystallinestate in which crystal grains of 0.5 nm to 20 nm can be observed isreferred to as microcrystal. Note that when using a non-crystallinesemiconductor film, the semiconductor layer 1714 may be formed using asputtering method, a CVD method, or the like, and when using acrystalline semiconductor film, the semiconductor layer 1714 may beformed by, for example, forming and then crystallizing a non-crystallinesemiconductor film. If necessary, a slight amount of an impurity element(such as phosphorus, arsenic, or boron) may be contained in thesemiconductor layer 1714 in addition to the above main component inorder to control a threshold voltage of a transistor.

Next, a gate insulating film 1715 is formed to cover the semiconductorlayer 1714. The gate insulating film 1715 is formed of a single layer ora plurality of stacked films using, for example, silicon oxide, siliconnitride, silicon nitride oxide, or the like. Note that a CVD method, asputtering method, or the like can be used as a film formation method.

Then, a gate electrode 1716 is formed over the semiconductor layer 1714with the gate insulating film 1715 interposed therebetween. The gateelectrode 1716 may be formed of a single layer or may be formed bystacking a plurality of metal films. Note that the gate electrode can beformed using a metal element selected from among tantalum (Ta), tungsten(W), titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), andchromium (Cr) or an alloy or compound material containing the element asits main component. For example, the gate electrode may include a firstconductive film and a second conductive film using tantalum nitride(TaN) as a first conductive layer and tungsten (W) as a secondconductive layer.

Next, an impurity which imparts n-type or p-type conductivity isselectively added to the semiconductor layer 1714 using as a mask thegate electrode 1716 or a resist which is formed into a desired shape. Inthis manner, a channel formation region and an impurity region(including a source region, a drain region, a GOLD region, and an LDDregion) are formed in the semiconductor layer 1714. In addition, thetransistor 1713 can be formed as either an n-channel transistor or ap-channel transistor depending on the conductivity type of an impurityelement to be added.

Note that in order to form an LDD region 1720 in a self-aligned mannerin FIG. 17, a silicon compound, for example, a silicon oxide film, asilicon nitride film, or a silicon oxynitride film is formed to coverthe gate electrode 1716 and then etched back to form a sidewall 1717.After that, a source region 1718, a drain region 1719, and an LDD region1720 can be formed by adding an impurity which imparts conductivity tothe semiconductor layer 1714. Therefore, the LDD region 1720 is locatedbelow the sidewall 1717. Note that the sidewall 1717 is provided to formthe LDD region 1720 in a self-aligned manner, and does not necessarilyneed to be provided. Note that phosphorus, arsenic, boron, or the likeis used as the impurity which imparts conductivity.

Next, a first interlayer insulating film 1730 is formed by stacking afirst insulating film 1721 and a second insulating film 1722 to coverthe gate electrode 1716. As the first insulating film 1721 and thesecond insulating film 1722, an inorganic insulating film such as asilicon oxide film, a silicon nitride film, or a silicon oxynitride(SiO_(x)N_(y)) film or an organic resin film (a photosensitive ornon-photosensitive organic resin film) with a low dielectric constantcan be used. Alternatively, a film containing siloxane may be used. Notethat siloxane is a material in which a skeleton is formed by the bond ofsilicon (Si) and oxygen (O), and an organic group (such as an alkylgroup or aromatic hydrocarbon) is used as a substituent. Further, afluoro group may be contained as a substituent.

Note that insulating films made of the same material may be used as thefirst insulating film 1721 and the second insulating film 1722. In thisembodiment mode, the first interlayer insulating film 1730 has a stackedstructure of two layers; however, it may be a single layer or have astacked structure of three or more layers.

Note that the first insulating film 1721 and the second insulating film1722 may be formed using a sputtering method, a CVD method, a spincoating method, or the like, and may be formed by a coating method inthe case of using an organic resin film or a film containing siloxane.

After that, source and drain electrodes 1723 are formed over the firstinterlayer insulating film 1730. Note that the source and drainelectrodes 1723 are connected to the source region 1718 and the drainregion 1719 through contact holes, respectively.

Note that the source and drain electrodes 1723 can be formed using ametal such as silver (Ag), gold (Au), copper (Cu), nickel (Ni), platinum(Pt), palladium (Pd), iridium (Ir), rhodium (Rh), tungsten (W), aluminum(Al), tantalum (Ta), molybdenum (Mo), cadmium (Cd), zinc (Zn), iron(Fe), titanium (Ti), silicon (Si), germanium (Ge), zirconium (Zr), orbarium (Ba), an alloy thereof, metal nitride thereof, or stacked filmsthereof.

Next, a second interlayer insulating film 1731 is formed to cover thesource and drain electrodes 1723. As the second interlayer insulatingfilm 1731, an inorganic insulating film, a resin film, or a stackedlayer thereof can be used. As the inorganic insulating film, a siliconnitride film, a silicon oxide film, a silicon oxynitride film, or astacked layer thereof can be used. For the resin film, polyimide,polyamide, acrylic, polyimide amide, epoxy, or the like can be used.

A pixel electrode 1724 is formed over the second interlayer insulatingfilm 1731. Next, an insulator 1725 is formed to cover an end portion ofthe pixel electrode 1724. The insulator 1725 is formed to have a curvedsurface with curvature at an upper end or a lower end thereof in orderto favorably form a layer 1726 containing a light emitting substancelater. For example, in the case of using positive photosensitive acrylicas a material of the insulator 1725, the insulator 1725 is preferablyformed to have a curved surface with a curvature radius (0.2 μm to 3 μm)only at an upper end. Either a negative resist which becomes insolublein an etchant by light irradiation or a positive resist which becomessoluble in an etchant by light irradiation can be used as the insulator1725. Further, an inorganic material such as silicon oxide or siliconoxynitride as well as an organic material can be used as a material ofthe insulator 1725.

Next, a layer 1726 containing a light emitting substance and an oppositeelectrode 1727 are formed over the pixel electrode 1724 and theinsulator 1725.

Note that a light emitting element 1728 is formed in a region where thelayer 1726 containing a light emitting substance is sandwiched betweenthe pixel electrode 1724 and the opposite electrode 1727.

Next, the detail of the light emitting element 1728 is explained withreference to FIGS. 18A and 18B. Note that the pixel electrode 1724 andthe opposite electrode 1727 in FIG. 17 correspond to a pixel electrode1801 and an opposite electrode 1802 in FIGS. 18A and 18B, respectively.In FIG. 18A, the pixel electrode is an anode and the opposite electrodeis a cathode.

As shown in FIG. 18A, a hole injection layer 1811, a hole transportlayer 1812, an electron transport layer 1814, an electron injectionlayer 1815, and the like as well as a light emitting layer 1813 areprovided between the pixel electrode 1801 and the opposite electrode1802. These layers are stacked so that holes are injected from the pixelelectrode 1801 side and electrons are injected from the oppositeelectrode 1802 side when applying a voltage to set a potential of thepixel electrode 1801 to be higher than a potential of the oppositeelectrode 1802.

In such a light emitting element, the holes injected from the pixelelectrode 1801 and the electrons injected from the opposite electrode1802 are recombined in the light emitting layer 1813 to excite a lightemitting substance. Then, the excited light emitting substance emitslight when returning to a ground state. Note that it is acceptable aslong as the light emitting substance is a substance which can provideluminescence (electroluminescence).

There is no particular limitation on the substance for forming the lightemitting layer 1813, and the light emitting layer may be formed of onlya light emitting substance.

However, when concentration quenching occurs, the light emitting layeris preferably a layer formed using a substance (host) having a largerenergy gap than that of the light emitting substance, in which the lightemitting substance is mixed so as to be dispersed. This can preventconcentration quenching of the light emitting substance. Note that theenergy gap refers to an energy difference between the lowest unoccupiedmolecular orbital (LUMO) level and the highest occupied molecularorbital (HOMO) level.

In addition, there is no particular limitation on the light emittingsubstance, and a substance which can emit light with a desired emissionwavelength may be used. For example, in order to obtain red lightemission, a substance which exhibits light emission having a peak of anemission spectrum at 600 nm to 680 nm can be used, such as4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJTI),4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJT),4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCJTB), perifanthene, or2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethbenyl]benzene.

In order to obtain green light emission, a substance which exhibitslight emission having a peak of an emission spectrum at 500 nm to 550 nmcan be used, such as N,N′-dimethylquinacridon (abbr.: DMQd), coumarin 6,coumarin 545T, tris(8-quinolinolato)aluminum (abbr.: Alq), orN,N′-diphenylquinacridon (DPQd). In order to obtain blue light emission,a substance which exhibits light emission having a peak of an emissionspectrum at 420 nm to 500 nm can be used, such as9,10-bis(2-naphthyl)-tert-butylanthracene (abbr.: t-BuDNA),9,9′-bianthryl, 9,10-diphenylanthracene (abbr.: DPA),9,10-bis(2-naphthyl)anthracene (abbr.: DNA),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbr.: BGaq),or bis(2-methyl-8-quinolinoato)-4-phenylphenolato-aluminum (abbr.:BAlq).

There is no particular limitation on the substance which is used fordispersing the light emitting substance, and for example, an anthracenederivative such as 9,10-di(2-naphthyl)-2-tert-butylanthracene (abbr.:t-BuDNA), a carbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl(abbr.: CBP), a metal complex such asbis[2-(2-hydroxyphenyl)pyridinato]zinc (abbr.: Znppi) orbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbr.: ZnBOX), or the likecan be used.

Although an anode material for forming the pixel electrode 1801 is notparticularly limited, a metal, an alloy, a conductive compound, amixture thereof, or the like having a high work function (a workfunction of 4.0 eV or higher) is preferably used.

As a specific example of such an anode material, oxide of a metalmaterial such as indium tin oxide (abbr.: ITO), ITO containing siliconoxide, or indium zinc oxide (abbr.: IZO) formed using a target in whichindium oxide is mixed with zinc oxide (ZnO) of 2 wt % to wtA can begiven. Further, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd), nitride of a metal material (for example, TiN), or thelike can be given.

On the other hand, as a substance for forming the opposite electrode1802, a metal, an alloy, a conductive compound, a mixture thereof, orthe like having a low work function (a work function of 3.8 eV or lower)can be used. As a specific example of such a cathode material, anelement belonging to Group 1 or 2 of the Periodic Table, that is, analkali metal such as lithium (Li) or cesium (Cs), an alkaline earthmetal such as magnesium (Mg), calcium (Ca), or strontium (Sr), or analloy containing these (Mg:Ag, Al:Li) can be given. In addition, byproviding a layer having an excellent electron injection propertybetween the opposite electrode 1802 and the light emitting layer 1813 soas to be stacked with the opposite electrode, various conductivematerials including the materials described as the material of the pixelelectrode 1801 such as Al, Ag, ITO, or ITO containing silicon oxide canbe used for the opposite electrode 1802 regardless of the magnitude ofthe work function. Alternatively, a similar effect can be obtained byforming the electron injection layer 1815 to be described later using amaterial having a particularly excellent electron injecting function.

Note that in order to extract light emission to outside, it ispreferable that either or both the pixel electrode 1801 and the oppositeelectrode 1802 are transparent electrodes made of ITO or the like or areformed with a thickness of several to several tens nm so as to be ableto transmit visible light.

The hole transport layer 1812 is provided between the pixel electrode1801 and the light emitting layer 1813 as shown in FIG. 18A. The holetransport layer is a layer functioning to transport holes injected fromthe pixel electrode 1801 to the light emitting layer 1813. By providingthe hole transport layer 1812 and separating the pixel electrode 1801and the light emitting layer 1813 from each other, light emission can beprevented from being quenched due to metal.

Note that the hole transport layer 1812 is preferably formed using asubstance having an excellent hole transport property, and particularly,a substance having a hole mobility of 1×10⁴ cm²Ns or more. Note that thesubstance having an excellent hole transport property refers to asubstance having a higher mobility of holes than that of electrons. As aspecific example of a substance which can be used for forming the holetransport layer 1812, 4,4′-bis[N-(l-naphthyl)-N-phenylamino]bipbenyl(abbr.: NPB), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbr.:TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbr.: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbr.:MTDATA), 4,4′-bis{N-[4-(NN-di-m-tolylamino)penyl]-N-phenylamino}biphenyl(abbr.: DNTPD), 1,3,5-tris[N,N-di(m-tolyl)amino]benzene (abbr.:m-MTDAB), 4,4′,4″-tris(Ncarbazolyl)tripheylamine (abbr.: TCTA),phthalocyanine (abbr.: H₂Pc), copper phthalocyanine (abbr.: CuPc),vanadyl phthalocyanine (abbr.: VOPc), or the like can be given. Inaddition, the hole transport layer 1812 may be a layer having amultilayer structure which is formed by combining two or more layersformed of the above-described substances.

Further, the electron transport layer 1814 may be provided between theopposite electrode 1802 and the light emitting layer 1813 as shown inFIG. 18A. Here, the electron transport layer is a layer functioning totransport electrons injected from the opposite electrode 1802 to thelight emitting layer 1813. By providing the electron transport layer1814 and separating the opposite electrode 1802 and the light emittinglayer 1813 from each other, light emission can be prevented from beingquenched due to metal.

There is no particular limitation on the material of the electrontransport layer 1814, and the electron transport layer 1814 can beformed of a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum (abbr.:Alq), tris(5-methyl-8-quinolinolato)aluminum (abbr.: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbr.: BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbr.: BAIq),or the like. Alternatively, it may be formed of a metal complex havingan oxazole-based or thiazole-based ligand such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbr.: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbr.: Zn(BTZ)₂), or thelike. Further, it may be formed using2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbr.: PBD),3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbr.:OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbr.: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbr.: p-EtTAZ), bathophenanthroline (abbr.: BPhen), bathocuproin(abbr.: BCP), or the like.

The electron transport layer 1814 is preferably formed using a substancehaving a higher mobility of electrons than that of holes as describedabove. In addition, the electron transport layer 1814 is preferablyformed using a substance having an electron mobility of 10⁻⁶ cm²Ns ormore. Note that the electron transport layer 1814 may be a layer havinga multilayer structure which is formed by combining two or more layersformed of the above-described substances.

Moreover, the hole injection layer 1811 may be provided between thepixel electrode 1801 and the hole transport layer 1812 as shown in FIG.18A. Here, the hole injection layer refers to a layer functioning topromote hole injection from an electrode functioning as an anode to thehole transport layer 1812.

There is no particular limitation on the material of the hole injectionlayer 1811, and the hole injection layer 1811 can be formed of metaloxide such as molybdenum oxide (MoO_(X)), vanadium oxide (VO_(X)),ruthenium oxide (RuO_(X)), tungsten oxide (WO_(X)), or manganese oxide(MnO_(X)). Alternatively, the hole injection layer 1811 can be formed ofa phthalocyanine-based compound such as phthalocyanine (abbr.: H₂Pc) orcopper phthalocyanine (CuPc), an aromatic amine-based compound such as4,4-bis(N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylamino)biphenyl (abbr.:DNTPD), a high molecular compound such as a poly(ethylenedioxythiophene)/poly(styrenesulfonic acid) aqueous solution (PEDOT/PSS),or the like.

In addition, a mixture of the metal oxide and a substance having anexcellent hole transport property may be provided between the pixelelectrode 1801 and the hole transport layer 1812. Since such a layerdoes not cause an increase in drive voltage even when thickened, opticaldesign using a microcavity effect or a light interference effect can beperformed by adjusting the thickness of the layer. Therefore, ahigh-quality light emitting element with excellent color purity and fewcolor changes depending on a viewing angle can be manufactured. Inaddition, a film thickness can be set so as to prevent short circuit ofthe pixel electrode 1801 and the opposite electrode 1802 due to theinfluence of unevenness generated at the time of film formation on thesurface of the pixel electrode 1801 and minute residue remaining on thesurface of the electrode.

In addition, the electron injection layer 1815 may be provided betweenthe opposite electrode 1802 and the electron transport layer 1814 asshown in FIG. 18A. Here, the electron injection layer is a layerfunctioning to promote electron injection from an electrode functioningas a cathode to the electron transport layer 1814. Note that when theelectron transport layer is not particularly provided, electroninjection to the light emitting layer may be supported by providing theelectron injection layer between the electrode functioning as a cathodeand the light emitting layer.

There is no particular limitation on the material of the electroninjection layer 1815, and the electron injection layer 1815 can beformed using a compound of alkali metal or alkaline earth metal, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂). Alternatively, a mixture of a substance having an excellentelectron transport property such as Alq or4,4-bis(5-methylbenzoxazol-2-yl)stilbene (BzOs) and alkali metal oralkaline earth metal such as magnesium or lithium can be used for theelectron injection layer 1815.

Note that each of the hole injection layer 1811, the hole transportlayer 1812, the light emitting layer 1813, the electron transport layer1814, and the electron injection layer 1815 may be formed by any methodsuch as an evaporation method, an ink-jet method, or a coating method.In addition, the pixel electrode 1801 or the opposite electrode 1802 mayalso be formed using any method such as a sputtering method or anevaporation method.

In addition, a layer structure of a light emitting element is notlimited to that shown in FIG. 18A, and may be manufactured by forminglayers sequentially from an electrode serving as a cathode as shown inFIG. 18B. In other words, the pixel electrode 1801 may be a cathode, andthe electron injection layer 1815, the electron transport layer 1814,the light emitting layer 1813, the hole transport layer 1812, the holeinjection layer 1811, and the opposite electrode 1802 may be stacked inthis order over the pixel electrode 1801. Note that the oppositeelectrode 1802 functions as an anode.

Note that the light emitting element is described to have a single lightemitting layer; however, the light emitting element may include aplurality of light emitting layers.

White light can be obtained by providing a plurality of light emittinglayers and mixing light emissions from the respective light emittinglayers. For example, in the case of a light emitting element includingtwo light emitting layers, a spacing layer, and a layer which generatesholes and a layer which generates electrons are preferably providedbetween a first light emitting layer and a second light emitting layer.This structure enables the light emitted from the respective lightemitting layers to outside to be visually mixed and perceived as whitelight. Thus, white light can be obtained.

Light emission is extracted to outside through either or both the pixelelectrode 1724 and the opposite electrode 1727 in FIG. 17. Accordingly,either or both the pixel electrode 1724 and the opposite electrode 1727are formed of a light-transmitting substance.

When only the opposite electrode 1727 is formed of a light-transmittingsubstance, light emission is extracted from a side opposite to thesubstrate through the opposite electrode 1727 as shown in FIG. 19A. Whenonly the pixel electrode 1724 is formed of a light-transmittingsubstance, light emission is extracted from the substrate side throughthe pixel electrode 1724 as shown in FIG. 19B. When both the pixelelectrode 1724 and the opposite electrode 1727 are formed of alight-transmitting substance, light emission is extracted from both thesubstrate side and the opposite side through the pixel electrode 1724and the opposite electrode 1727 as shown in FIG. 19C.

Next, a transistor having a staggered structure using a non-crystallinesemiconductor film for the semiconductor layer of the transistor 1713 isexplained. Fragmentary sectional views of pixels are shown in FIGS. 20Aand 20B. Note that in each of FIGS. 20A and 20B, a transistor having astaggered structure is shown and a capacitor included in a pixel is alsoexplained.

As shown in FIG. 20A, a base film 2012 is formed over a substrate 2011.Further, a pixel electrode 2013 is formed over the base film 2012. Inaddition, a first electrode 2014 is formed of the same material and inthe same layer as the pixel electrode 2013.

Further, a wiring 2015 and a wiring 2016 are formed over the base film2012, and an end of the pixel electrode 2013 is covered with the wiring2015. An n-type semiconductor layer 2017 and an n-type semiconductorlayer 2018 each having n-type conductivity are formed over the wiring2015 and the wiring 2016. In addition, a semiconductor layer 2019 isformed over the base film 2012 and between the wiring 2015 and thewiring 2016. A part of the semiconductor layer 2019 is extended so as tooverlap with the n-type semiconductor layer 2017 and the n-typesemiconductor layer 2018. Note that this semiconductor layer is formedof a non-crystalline semiconductor film made of an amorphoussemiconductor such as amorphous silicon (a-Si:H), a semi-amorphoussemiconductor, a microcrystalline semiconductor, or the like. Inaddition, a gate insulating film 2020 is formed over the semiconductorlayer 2019. An insulating film 2021 made of the same material and in thesame layer as the gate insulating film 2020 is also formed over thefirst electrode 2014.

Furthermore, a gate electrode 2022 is formed over the gate insulatingfilm 2020; thus, a transistor 2025 is formed. In addition, a secondelectrode 2023 made of the same material and in the same layer as thegate electrode 2022 is formed over the first electrode 2014 with theinsulating film 2021 interposed therebetween, and a capacitor 2024 isformed in which the insulating film 2021 is sandwiched between the firstelectrode 2014 and the second electrode 2023. An interlayer insulatingfilm 2026 is formed to cover the end of the pixel electrode 2013, thetransistor 2025, and the capacitor 2024.

A layer 2027 containing a light emitting substance and an oppositeelectrode 2028 are formed over the interlayer insulating film 2026 andthe pixel electrode 2013 located in an opening of the interlayerinsulating film 2026, and a light emitting element 2029 is formed in aregion where the layer 2027 containing a light emitting substance issandwiched between the pixel electrode 2013 and the opposite electrode2028.

The first electrode 2014 shown in FIG. 20A may be formed of the samematerial and in the same layer as the wirings 2015 and 2016 as shown inFIG. 20B, and a capacitor 2031 may be formed in which the insulatingfilm 2021 is sandwiched between the first electrode 2030 and the secondelectrode 2023. Although an n-channel transistor is used as thetransistor 2025 in FIGS. 20A and 20B, a p-channel transistor may beused.

Materials of the substrate 2011, the base film 2012, the pixel electrode2013, the gate insulating film 2020, the gate electrode 2022, theinterlayer insulating film 2026, the layer 2027 containing a lightemitting substance, and the opposite electrode 2028 may be similar tothose of the substrate 1711, the base film 1712, the pixel electrode1724, the gate insulating film 1715, the gate electrode 1716, theinterlayer insulating films 1730 and 1731, the layer 1726 containing alight emitting substance, and the opposite electrode 1727 shown in FIG.17. The wiring 2015 and the wiring 2016 may be formed using similarmaterials to those of the source and drain electrodes 1723 in FIG. 17.

Next, fragmentary sectional views of pixels each having a transistorwith a structure in which a gate electrode is sandwiched between asubstrate and a semiconductor layer, in other words, a bottom-gatetransistor in which a gate electrode is located below a semiconductorlayer are shown in FIGS. 21A and 21B as other structures of a transistorusing a non-crystalline semiconductor film for a semiconductor layer.

A base film 2112 is formed over a substrate 2111. A gate electrode 2113is formed over the base film 2112. In addition, a first electrode 2114is formed of the same material and in the same layer as the gateelectrode 2113. A material of the gate electrode 2113 may bepolycrystalline silicon to which phosphorus is added or silicide that isa compound of metal and silicon as well as the material used for thegate electrode 1716 shown in FIG. 17.

A gate insulating film 2115 is formed to cover the gate electrode 2113and the first electrode 2114.

A semiconductor layer 2116 is formed over the gate insulating film 2115.A semiconductor layer 2117 made of the same material and in the samelayer as the semiconductor layer 2116 is formed over the first electrode2114. Note that this semiconductor layer is formed of a non-crystallinesemiconductor film made of an amorphous semiconductor such as amorphoussilicon (a-Si:H), a semi-amorphous semiconductor, a microcrystallinesemiconductor, or the like.

An n-type semiconductor layer 2118 and an n-type semiconductor layer2119 each having n-type conductivity are formed over the semiconductorlayer 2116, and an n-type semiconductor layer 2120 is formed over thesemiconductor layer 2117.

A wiring 2121 and a wiring 2122 are formed over the n-type semiconductorlayer 2118 and the n-type semiconductor layer 2119, respectively, and atransistor 2129 is formed. A conductive layer 2123 made of the samematerial and in the same layer as the wiring 2121 and the wiring 2122 isformed over the n-type semiconductor layer 2120, and this conductivelayer 2123, the n-type semiconductor layer 2120, and the semiconductorlayer 2117 form a second electrode. Note that a capacitor 2130 is formedin which the gate insulating film 2115 is sandwiched between this secondelectrode and the first electrode 2114.

One end of the wiring 2121 is extended, and a pixel electrode 2124 isformed on the extended portion of the wiring 2121.

An insulator 2125 is formed to cover an end of the pixel electrode 2124,the transistor 2129, and the capacitor 2130.

A layer 2126 containing a light emitting substance and an oppositeelectrode 2127 are formed over the pixel electrode 2124 and theinsulator 2125, and a light emitting element 2128 is formed in a regionwhere the layer 2126 containing a light emitting substance is sandwichedbetween the pixel electrode 2124 and the opposite electrode 2127.

The semiconductor layer 2117 and the n-type semiconductor layer 2120which serve as a part of the second electrode of the capacitor 2130 donot particularly need to be provided. In other words, a capacitor may beformed in which the conductive layer 2123 is used as the secondelectrode and the gate insulating film 2115 is sandwiched between thefirst electrode 2114 and the conductive layer 2123.

Although an n-channel transistor is used as the transistor 2129, ap-channel transistor may be used.

Note that a capacitor 2132 having a structure in which the gateinsulating film 2115 is sandwiched between the first electrode 2114 anda second electrode 2131 made of the same material and in the same layeras the pixel electrode 2124 as shown in FIG. 21B can also be formed byforming the pixel electrode 2124 before forming the wiring 2121 in FIG.21A.

Although an inverted staggered transistor with a channel etch structureis described, it goes without saying that a transistor with a channelprotective structure may be used. Next, the case of a transistor with achannel protective structure is explained with reference to FIGS. 22Aand 22B. Note that a common reference numeral is used in FIGS. 22A and22B to denote a similar component to that in FIGS. 21A and 21B.

A transistor 2201 with a channel protective structure shown in FIG. 22Ais different from the transistor 2129 with a channel etch structureshown in FIG. 21A in that an insulator 2202 serving as an etching maskis provided over a region of the semiconductor layer 2116 in which achannel is formed.

Similarly, a transistor 2201 with a channel protective structure shownin FIG. 22B is different from the transistor 2129 with a channel etchstructure shown in FIG. 21B in that an insulator 2202 serving as anetching mask is provided over a region of the semiconductor layer 2116in which a channel is formed.

Manufacturing cost can be reduced by using a non-crystallinesemiconductor film for a semiconductor layer of a transistor included inthe pixel of the present invention.

Note that the materials explained with reference to FIG. 17 can be usedas respective materials.

Structures of a transistor and a capacitor are not limited to thosedescribed above, and transistors and capacitors having variousstructures can be used.

A crystalline semiconductor film made of polysilicon (p-Si:H) or thelike as well as a non-crystalline semiconductor film made of anamorphous semiconductor such as amorphous silicon (a-Si:H), asemi-amorphous semiconductor, a microcrystalline semiconductor, or thelike may be used for a semiconductor layer of a transistor.

FIG. 23 shows a fragmentary sectional view of a pixel including atransistor using a crystalline semiconductor film for a semiconductorlayer, which is explained below. Note that a transistor 2318 shown inFIG. 23 is the multi-gate transistor shown in FIG. 10.

As shown in FIG. 23, a base film 2302 is formed over a substrate 2301,and a semiconductor layer 2303 is formed thereover. Note that thesemiconductor layer 2303 is formed by patterning a crystallinesemiconductor film into a desired shape.

An example of a method for manufacturing the crystalline semiconductorfilm is described below. First, an amorphous silicon film is formed overthe substrate 2301 by a sputtering method, a CVD method, or the like. Afilm formation material does not need to be limited to an amorphoussilicon film, and a non-crystalline semiconductor film made of anamorphous semiconductor, a semi-amorphous semiconductor, amicrocrystalline semiconductor, or the like is acceptable. In addition,a compound semiconductor film having an amorphous structure such as anamorphous silicon germanium film may be used.

Then, the amorphous silicon film obtained is crystallized using athermal crystallization method, a laser crystallization method, athermal crystallization method using a catalytic element such as nickel,or the like, thereby obtaining a crystalline semiconductor film. Notethat crystallization may be performed by a combination of thesecrystallization methods.

In the case of forming a crystalline semiconductor film by a thermalcrystallization method, a heating furnace, laser irradiation, RTA (RapidThermal Annealing), or a combination thereof can be used.

When the crystalline semiconductor film is formed by a lasercrystallization method, a continuous wave laser beam (CW laser beam) ora pulsed laser beam can be used. As a laser beam that can be used here,a laser beam emitted from one or more kinds of a gas laser such as an Arlaser, a Kr laser, or an excimer laser, a laser using, as a medium,single-crystal YAG, YVO₄, forsterite (Mg₂SiO₄), YAIO₃, or GdVO₄, orpolycrystalline (ceramic) YAQ Y₂O₃, YVO₄, YAIO₃, or GdVO₄ doped with oneor more of Nd, Yb, Cr, T, Ho, Er, Tm, and Ta as a dopant; a glass laser,a ruby laser, an alexandrite laser, a Ti:sapphire laser, a copper vaporlaser; and a gold vapor laser can be used. A crystal having a largegrain diameter can be obtained by irradiation with the fundamental waveof the above laser beam or a second harmonic to a fourth harmonic of thelaser beam. For example, the second harmonic (532 nm) or the thirdharmonic (355 nm) of a Nd:YVO₄ laser (the fundamental wave: 1064 nm) canbe used. At this time, an energy density of the laser is required to beabout 0.01 MW/cm² to 100 MW/cm² (preferably, 0.1 MW/cm² to 10 MW/cm²). Ascanning rate is set to about 10 cm/sec to 2000 cm/sec for irradiation.

Note that a laser using, as a medium, single-crystal YAQ, YVO₄,forsterite (Mg₂SiO₄), YAIO₃, or GdVO₄, or polycrystalline (ceramic) YAQ,Y₂O₃, YVO₄, YAIO₃, or GdVO₄ doped with one or more of Nd, Yb, Cr, Ti,Ho, Er, Tm, and Ta as a dopant; an Ar ion laser; or a Ti:sapphire lasercan be a CW laser. Alternatively, it can be pulsed at a repetition rateof 10 MHz or more by performing Q-switch operation, mode locking, or thelike. When a laser beam is pulsed at a repetition rate of 10 MHz ormore, the semiconductor film is irradiated with the following pulsedlaser after being melted by a preceding laser and before beingsolidified. Therefore, unlike the case of using a pulsed laser having alow repetition rate, the interface between the solid phase and theliquid phase can be moved continuously in the semiconductor film, sothat crystal grains grown continuously in the scanning direction can beobtained.

In the case of forming a crystalline semiconductor film by a thermalcrystallization method using a catalytic element such as nickel, it ispreferable to perform gettering treatment for removing the catalyticelement such as nickel after crystallization.

By the above-described crystallization, a crystallized region is formedin a part of the amorphous semiconductor film. This partly crystallizedcrystalline semiconductor film is patterned into a desired shape,thereby forming an island-shaped semiconductor film. This semiconductorfilm is used for the semiconductor layer 2303 of the transistor.

The crystalline semiconductor layer is used for a channel formationregion 2304 and an impurity region 2305 serving as a source region or adrain region of the transistor 2318 and also for a semiconductor layer2306 and an impurity region 2308 serving as a lower electrode of acapacitor 2319. Note that the impurity region 2308 does not particularlyneed to be provided. Channel doping may be performed to the channelformation region 2304 and the semiconductor layer 2306.

Next, a gate insulating film 2309 is formed over the semiconductor layer2303 and the lower electrode of the capacitor 2319. Further, a gateelectrode 2310 is formed over the semiconductor layer 2303 with the gateinsulating film 2309 interposed therebetween, and an upper electrode2311 made of the same material and in the same layer as the gateelectrode 2310 is formed over the semiconductor layer 2306 of thecapacitor 2319 with the gate insulating film 2309 interposedtherebetween. In this manner, the transistor 2318 and the capacitor 2319are manufactured.

Next, an interlayer insulating film 2312 is formed to cover thetransistor 2318 and the capacitor 2319, and a wiring 2313 is formed overthe interlayer insulating film 2312 so as to be in contact with theimpurity region 2305 through a contact hole. Then, a pixel electrode2314 is formed in contact with the wiring 2313 and over the interlayerinsulating film 2312, and an insulator 2315 is formed to cover an end ofthe pixel electrode 2314 and the wiring 2313. Further, a layer 2316containing a light emitting substance and an opposite electrode 2317 areformed over the pixel electrode 2314, and a light emitting element 2320is formed in a region where the layer 2316 containing a light emittingsubstance is sandwiched between the pixel electrode 2314 and theopposite electrode 2317.

A fragmentary cross section of a pixel including a bottom-gatetransistor using a crystalline semiconductor film made of polysilicon(p-Si:H) or the like for a semiconductor layer is shown in FIG. 24.

A base film 2402 is formed over a substrate 2401, and a gate electrode2403 is formed thereover. In addition, a first electrode 2404 of acapacitor 2423 is formed of the same material and in the same layer asthe gate electrode 2403.

A gate insulating film 2405 is formed to cover the gate electrode 2403and the first electrode 2404.

A semiconductor layer is formed over the gate insulating film 2405. Notethat the semiconductor layer is formed by crystallizing anon-crystalline semiconductor film made of an amorphous semiconductor, asemi-amorphous semiconductor, a microcrystalline semiconductor, or thelike using a thermal crystallization method, a laser crystallizationmethod, a thermal crystallization method using a catalytic element suchas nickel, or the like and patterning the crystallized semiconductorfilm into a desired shape.

Note that a channel formation region 2406, an LDD region 2407, and animpurity region 2408 serving as a source region or a drain region of atransistor 2422, and a region 2409 serving as a second electrode, andimpurity regions 2410 and 2411 of the capacitor 2423 are formed usingthe semiconductor layer. Note that the impurity regions 2410 and 2411are not necessarily required to be provided. In addition, an impuritymay be added to the channel formation region 2406 and the region 2409.

Note that the capacitor 2423 has a structure in which the gateinsulating film 2405 is sandwiched between the first electrode 2404 andthe second electrode including the region 2409 formed of thesemiconductor layer and the like.

Next, a first interlayer insulating film 2412 is formed to cover thesemiconductor layer, and a wiring 2413 is formed over the firstinterlayer insulating film 2412 so as to be in contact with the impurityregion 2408 through a contact hole.

An opening 2415 is formed in the first interlayer insulating film 2412.A second interlayer insulating film 2416 is formed to cover thetransistor 2422, the capacitor 2423, and the opening 2415, and a pixelelectrode 2417 is formed over the second interlayer insulating film 2416so as to be connected to the wiring 2413 through a contact hole. Inaddition, an insulator 2418 is formed to cover an end of the pixelelectrode 2417. Then, a layer 2419 containing a light emitting substanceand an opposite electrode 2420 are formed over the pixel electrode 2417,and a light emitting element 2421 is formed in a region where the layer2419 containing a light emitting substance is sandwiched between thepixel electrode 2417 and the opposite electrode 2420. Note that theopening 2415 is located below the light emitting element 2421. In otherwords, since the first interlayer insulating film 2412 has the opening2415, transmittance can be increased when light emission from the lightemitting element 2421 is extracted from the substrate side.

By using a crystalline semiconductor film for a semiconductor layer of atransistor included in the pixel of the present invention, it becomeseasier to form the scan line driver circuit 612 and the signal linedriver circuit 611 in FIG. 6 over the same substrate as the pixelportion 613, for example.

Note that a structure of a transistor using a crystalline semiconductorfilm for a semiconductor layer is not limited to that described above,and the transistor can have various structures. Note that the sameapplies to a capacitor. In this embodiment mode, the materials in FIG.17 can be appropriately used unless stated otherwise.

The transistor described in this embodiment mode can be used as thetransistor which controls a current value supplied to the light emittingelement in the pixel described in any of Embodiment Modes 1 to 7.Therefore, variation in current value caused by variation in thresholdvoltage of the transistor can be suppressed by operating the pixel asdescribed in any of Embodiment Modes 1 to 7. Accordingly, current inaccordance with luminance data can be supplied to a light emittingelement, and variation in luminance can be suppressed. In addition,power consumption can be reduced because operation is performed with anopposite electrode fixed at a constant potential.

In addition, since each pixel can emit light except in its addressperiod by applying such a pixel to the display device of FIG. 6, a ratioof a light emitting period to one frame period (that is, a duty ratio)can be significantly high and can be approximately 100%. Therefore, adisplay device with less luminance variation and a high duty ratio canbe provided.

In addition, since a threshold write period can be set to be long, athreshold voltage of a transistor which controls a current value flowingto a light emitting element can be written into a capacitor moreaccurately. Therefore, reliability as a display device is improved.

Embodiment Mode 9

In this embodiment mode, one mode of a display device of the presentinvention is explained with reference to FIGS. 25A and 25B.

FIG. 25A is a top view showing a display device, and FIG. 25B is an A-A′line cross sectional view (cross sectional view taken along a line A-A′)of FIG. 25A. The display device includes a signal line driver circuit2501, a pixel portion 2502, a first scan line driver circuit 2503, and asecond scan line driver circuit 2506 over a substrate 2510 which areindicated by dotted lines in the drawing. The display device alsoincludes a sealing substrate 2504 and a sealant 2505, and a portion ofthe display device surrounded by them is a space 2507.

Note that a wiring 2508 is a wiring for transmitting signals to beinputted to the first scan line driver circuit 2503, the second scanline driver circuit 2506, and the signal line driver circuit 2501 andreceives a video signal, a clock signal, a start signal, and the likethrough an FPC (Flexible Printed Circuit) 2509 that serves as anexternal input terminal. IC chips (semiconductor chips provided with amemory circuit, a buffer circuit, and the like) 2518 and 2519 aremounted by COG (Chip On Glass) or the like on a connection portion ofthe FPC 2509 and the display device. Note that only the FPC is shownhere, but a printed wiring board (PWB) may be attached to the FPC. Thedisplay device of the present invention includes not only a main body ofa display device but also a display device with an FPC or a PWB attachedthereto. In addition, it also includes a display device on which an ICchip or the like is mounted.

A cross-sectional structure is explained with reference to FIG. 25B. Thepixel portion 2502 and its peripheral driver circuits (the first scanline driver circuit 2503, the second scan line driver circuit 2506, andthe signal line driver circuit 2501) are formed over the substrate 2510;here, the signal line driver circuit 2501 and the pixel portion 2502 areshown.

Note that the signal line driver circuit 2501 includes transistors withsingle polarity such as n-channel transistors 2520 and 2521. It goeswithout saying that a p-channel transistor may be used or a CMOS circuitmay be formed using not only an n-channel transistor but also ap-channel transistor. In this embodiment mode, the display panel inwhich the peripheral driver circuits are formed over the same substrateas the pixel portion is described; however, the present invention is notlimited to this. All or part of the peripheral driver circuits may beformed on an IC chip or the like and mounted by COO or the like.

The pixel described in any of Embodiment Modes 1 to 7 is used for thepixel portion 2502. Note that FIG. 25B shows a transistor 2511 whichfunctions as a switch, a transistor 2512 which controls a current valuesupplied to a light emitting element, and a light emitting element 2528.Note that a first electrode of the transistor 2512 is connected to apixel electrode 2513 of the light emitting element 2528. In addition, aninsulator 2514 is formed to cover an end of the pixel electrode 2513.Here, the insulator 2514 is formed using a positive photosensitiveacrylic resin film.

The insulator 2514 is formed to have a curved surface with a curvatureat an upper end portion or a lower end portion thereof in order toobtain favorable coverage. For example, in the case of using positivephotosensitive acrylic as a material of the insulator 2514, theinsulator 2514 is preferably formed to have a curved surface with acurvature radius (0.2 μm to 3 μm) only at the upper end portion. Eithera negative resist which becomes insoluble in an etchant by lightirradiation or a positive resist which becomes soluble in an etchant bylight irradiation can be used as the insulator 2514.

A layer 2516 containing a light emitting substance and an oppositeelectrode 2517 are formed over the pixel electrode 2513. As long as thelayer 2516 containing a light emitting substance is provided with atleast a light emitting layer, there is no particular limitation onlayers other than the light emitting layer, which can be appropriatelyselected.

By attaching the sealing substrate 2504 to the substrate 2510 using thesealant 2505, a structure is obtained in which the light emittingelement 2528 is provided in the space 2507 surrounded by the substrate2510, the sealing substrate 2504, and the sealant 2505. Note that thereis also a case where the space 2507 is filled with the sealant 2505other than an inert gas (such as nitrogen or argon).

Note that an epoxy-based resin is preferably used as the sealant 2505.The material preferably allows as little moisture and oxygen as possibleto penetrate. As the sealing substrate 2504, a plastic substrate formedof FRP (Fiberglass-Reinforced Plastics), PVF (polyvinyl fluoride),Mylar, polyester, acrylic, or the like can be used other than a glasssubstrate or a quartz substrate.

Variation in luminance among pixels or fluctuation in luminance of apixel over time can be suppressed by using and operating any of thepixels described in Embodiment Modes 1 to 7 for the pixel portion 2502,and thus a display device with a higher duty ratio and higher qualitycan be obtained. In addition, power consumption can be reduced in thepresent invention because operation is performed with the oppositeelectrode fixed at a constant potential.

By forming the signal line driver circuit 2501, the pixel portion 2502,the first scan line driver circuit 2503, and the second scan line drivercircuit 2506 over the same substrate as shown in FIGS. 25A and 25B, costof the display device can be reduced. In this case, a manufacturingprocess can be simplified by using transistors with single polarity forthe signal line driver circuit 2501, the pixel portion 2502, the firstscan line driver circuit 2503, and the second scan line driver circuit2506. Accordingly, a further cost reduction can be achieved.

The display device of the present invention can be obtained as describedabove.

Note that the above-described structure is one example and a structureof the display device of the present invention is not limited to this.

Note that the structure of the display device may be that in which asignal line driver circuit 2601 is formed on an IC chip and the IC chipis mounted on a display device by COG or the like as shown in FIG. 26A.Note that a substrate 2600, a pixel portion 2602, a first scan linedriver circuit 2603, a second scan line driver circuit 2604, an FPC2605, an IC chip 2606, an IC chip 2607, a sealing substrate 2608, and asealant 2609 of FIG. 26A correspond to the substrate 2510, the pixelportion 2502, the first scan line driver circuit 2503, the second scanline driver circuit 2506, the FPC 2509, the IC chip 2518, the IC chip2519, the sealing substrate 2504, and the sealant 2505 in FIG. 25A,respectively.

In other words, only a signal line driver circuit of which high speedoperation is required is formed on an IC chip using a CMOS or the liketo reduce power consumption. In addition, higher-speed operation andlower power consumption can be achieved by using a semiconductor chipmade of a silicon wafer or the like as the IC chip.

Note that cost reduction can be achieved by forming the first scan linedriver circuit 2603 and the second scan line driver circuit 2604 overthe same substrate as the pixel portion 2602. A further cost reductioncan be achieved by forming the first scan line driver circuit 2603, thesecond scan line driver circuit 2604, and the pixel portion 2602 usingtransistors with single polarity. At the time, a decrease in outputpotential can be prevented by using boot trap circuits for the firstscan line driver circuit 2603 and the second scan line driver circuit2604. In addition, in the case of using amorphous silicon forsemiconductor layers of transistors included in the first scan linedriver circuit 2603 and the second scan line driver circuit 2604, thethreshold voltage of each transistor varies due to deterioration.Therefore, it is preferable to provide a function to correct thevariation.

Variations in luminance among pixels or fluctuation in luminance of apixel over time can be suppressed by using and operating any of thepixels described in Embodiment Modes 1 to 7 for the pixel portion 2602,and thus, a display device with a higher duty ratio and higher qualitycan be obtained. In addition, power consumption can be reduced in thepresent invention because operation is performed with an oppositeelectrode fixed at a constant potential. In addition, a substrate areacan be used efficiently by mounting an IC chip provided with afunctional circuit (a memory or a buffer) on a connection portion of theFPC 2605 and the substrate 2600.

In addition, a structure may be employed in which a signal line drivercircuit 2611, a first scan line driver circuit 2613, and a second scanline driver circuit 2614 corresponding to the signal line driver circuit2501, the first scan line driver circuit 2503, and the second scan linedriver circuit 2506 of FIG. 25A are formed on IC chips and the IC chipsare mounted on a display device by COO or the like as shown in FIG. 26B.Note that a substrate 2610, a pixel portion 2612, an FPC 2615, an ICchip 2616, an IC chip 2617, a sealing substrate 2618, and a sealant 2619of FIG. 26B correspond to the substrate 2510, the pixel portion 2502,the FPC 2509, the IC chip 2518, the IC chip 2519, the sealing substrate2504, and the sealant 2505 of FIG. 25A, respectively.

Cost reduction can be achieved by using a non-crystalline semiconductorfilm, for example, an amorphous silicon (a-Si:H) film for thesemiconductor layer of the transistor of the pixel portion 2612.Further, a large-sized display panel can also be manufactured.

Further, the first scan line driver circuit, the second scan line drivercircuit, and the signal line driver circuit are not necessarily providedin a row direction and a column direction of the pixels. For example, aperipheral driver circuit 2701 formed on an IC chip as shown in FIG. 27Amay have functions of the first scan line driver circuit 2613, thesecond scan line driver circuit 2614, and the signal line driver circuit2611 shown in FIG. 26B. Note that a substrate 2700, a pixel portion2702, an FPC 2704, an IC chip 2705, an IC chip 2706, a sealing substrate2707, and a sealant 2708 of FIG. 27A correspond to the substrate 2510,the pixel portion 2502, the FPC 2509, the IC chip 2518, the IC chip2519, the sealing substrate 2504, and the sealant 2505 of FIG. 25A,respectively.

Note that a schematic diagram illustrating the connection of wirings ofthe display device of FIG. 27A is shown in FIG. 27B. FIG. 27B shows asubstrate 2710, a peripheral driver circuit 2711, a pixel portion 2712,an FPC 2713, and an FPC 2714.

The FPC 2713 and the FPC 2714 input a signal and a power supplypotential from outside to the peripheral driver circuit 2711. Then, anoutput from the peripheral driver circuit 2711 is inputted to wirings ina row direction and a column direction connected to pixels included inthe pixel portion 2712.

In addition, in the case of using a white light emitting element as thelight emitting element, full color display can be realized by providingthe sealing substrate with color filters. The present invention can beapplied to such a display device. FIG. 28 shows an example of afragmentary sectional view of a pixel portion.

As shown in FIG. 28, a base film 2802 is formed over a substrate 2800; atransistor 2801 which controls a current value supplied to a lightemitting element is formed thereover, and a pixel electrode 2803 isformed in contact with a first electrode of the transistor 2801. A layer2804 containing a light emitting substance and an opposite electrode2805 are formed thereover.

Note that a portion where the layer 2804 containing a light emittingsubstance is sandwiched between the pixel electrode 2803 and theopposite electrode 2805 serves as the light emitting element. Note thatwhite light is emitted in FIG. 28. A red color filter 2806R, a greencolor filter 2806G, and a blue color filter 2806B are provided above thelight emitting elements respectively to achieve full-color display. Inaddition, a black matrix (also referred to as a “BM”) 2807 is providedto separate these color filters.

The display device of this embodiment mode can be appropriately combinedwith the structure described in Embodiment Mode 8 as well as those inEmbodiment Modes 1 to 7. In addition, a structure of a display device isnot limited to that described above, and the present invention can alsobe applied to a display device having another structure.

Embodiment Mode 10

The display device of the present invention can be applied to variouselectronic devices. Specifically, it can be applied to a display portionof an electronic device. Note that examples of electronic devices are asfollows: a camera such as a video camera or a digital camera, a goggletype display, a navigation system, an audio-reproducing device (caraudio, an audio component, or the like), a computer, a game machine, aportable information terminal (a mobile computer, a mobile phone, amobile game machine, an electronic book, or the like), animage-reproducing device having a recording medium (specifically, adevice for reproducing a recording medium such as a digital versatiledisc (DVD) and having a display for displaying the reproduced image),and the like.

FIG. 33A shows a display, which includes a chassis 3301, a support 3302,a display portion 3303, a speaker portion 3304, a video input terminal3305, and the like.

Note that the pixel described in any of Embodiment Modes 1 to 7 is usedfor the display portion 3303. According to the present invention,variation in luminance among pixels or fluctuation in luminance of apixel over time can be suppressed and a display including a displayportion with a higher duty ratio and higher quality can be obtained.

In addition, power consumption can be reduced in the present inventionbecause operation is performed with an opposite electrode fixed at aconstant potential. Note that the display includes in its category alldisplay devices used for displaying information, for example, for apersonal computer, for TV broadcast reception, for advertisementdisplay, or the like.

Note that while needs for an increase in display size have beenincreasing, an increase in price associated with the increase in displaysize has become an issue. Therefore, it is an essential task to reducemanufacturing cost and set the price of a high-quality product as low aspossible.

Since the pixel of the present invention can be manufactured usingtransistors with single polarity, the number of steps can be reduced andmanufacturing cost can be reduced. A process can be simplified and afurther cost reduction can be achieved by using a non-crystallinesemiconductor film, for example, an amorphous silicon (a-Si:H) film fora semiconductor layer of each transistor included in the pixel. In thiscase, a driver circuit on the periphery of a pixel portion is preferablyformed on an IC chip and the IC chip is mounted on a display panel byCOG (Chip On Glass) or the like. Note that a signal line driver circuitwith high operation speed may be formed on an IC chip, and a scan linedriver circuit with relatively low operation speed may be formed using acircuit including transistors with single polarity over the samesubstrate as a pixel portion.

FIG. 33B shows a camera, which includes a main body 3311, a displayportion 3312, an image receiving portion 3313, an operation key 3314, anexternal connection port 3315, a shutter 3316, and the like.

Note that the pixel described in any of Embodiment Modes 1 to 7 is usedfor the display portion 3312. According to the present invention,variation in luminance among pixels or fluctuation in luminance of apixel over time can be suppressed, and a camera including a displayportion with a higher duty ratio and higher quality can be obtained.

In addition, power consumption can be reduced in the present inventionbecause operation is performed with an opposite electrode fixed at aconstant potential.

In addition, competitive manufacturing of a digital camera or the likehas been intensified with an improvement in performance. Therefore, itis vital to set the price of a high-performance product as low aspossible.

Since the pixel of the present invention can be manufactured usingtransistors with single polarity, the number of steps can be reduced andmanufacturing cost can be reduced. A process can be simplified and afurther cost reduction can be achieved by using a non-crystallinesemiconductor film, for example, an amorphous silicon (a-Si:H) film fora semiconductor layer of each transistor included in the pixel. In thiscase, a driver circuit on the periphery of a pixel portion is preferablyformed on an IC chip and the IC chip is mounted on a display panel byCOO or the like. Note that a signal line driver circuit with highoperation speed may be formed on an IC chip, and a scan line drivercircuit with relatively low operation speed may be formed using acircuit including transistors with single polarity over the samesubstrate as a pixel portion.

FIG. 33C shows a computer, which includes a main body 3321, a chassis3322, a display portion 3323, a keyboard 3324, an external connectionport 3325, a pointing mouse 3326, and the like. Note that the pixeldescribed in any of Embodiment Modes 1 to 7 is used for the displayportion 3323. According to the present invention, variation in luminanceamong pixels or fluctuation in luminance of a pixel over time can besuppressed and a computer including a display portion with a higher dutyratio and higher quality can be obtained. In addition, power consumptioncan be reduced in the present invention because operation is performedwith an opposite electrode fixed at a constant potential. A costreduction can be achieved by using transistors with single polarity fortransistors included in the pixel portion and a non-crystallinesemiconductor film for semiconductor layers of the transistors.

FIG. 33D shows a mobile computer, which includes a main body 3331, adisplay portion 3332, a switch 3333, an operation key 3334, an infraredport 3335, and the like. Note that the pixel described in any ofEmbodiment Modes 1 to 7 is used for the display portion 3332. Accordingto the present invention, variation in luminance among pixels orfluctuation in luminance of a pixel over time can be suppressed and amobile computer including a display portion with a higher duty ratio andhigher quality can be obtained. In addition, power consumption can bereduced in the present invention because operation is performed with anopposite electrode fixed at a constant potential. A cost reduction canbe achieved by using transistors with single polarity for transistorsincluded in the pixel portion and a non-crystalline semiconductor filmfor semiconductor layers of the transistors.

FIG. 33E shows a portable image reproducing device provided with arecording medium (specifically, a DVD player), which includes a mainbody 3341, a chassis 3342, a display portion A 3343, a display portion B3344, a recording medium (DVD or the like) reading portion 3345, anoperation key 3346, a speaker portion 3347, and the like. The displayportion A 3343 mainly displays image information, and the displayportion B 3344 mainly displays character information. Note that thepixel described in any of Embodiment Modes 1 to 7 is used for thedisplay portion A 3343 and the display portion B 3344. According to thepresent invention, variation in luminance among pixels or fluctuation inluminance of a pixel over time can be suppressed and an imagereproducing device including a display portion with a higher duty ratioand higher quality can be obtained. In addition, power consumption canbe reduced in the present invention because operation is performed withan opposite electrode fixed at a constant potential. A cost reductioncan be achieved by using transistors with single polarity fortransistors included in the pixel portion and a non-crystallinesemiconductor film for semiconductor layers of the transistors.

FIG. 33F shows a goggle type display, which includes a main body 3351, adisplay portion 3352, an arm portion 3353, and the like. Note that thepixel described in any of Embodiment Modes 1 to 7 is used for thedisplay portion 3352. According to the present invention, variation inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed and a goggle type display including a display portionwith a higher duty ratio and higher quality can be obtained. Inaddition, power consumption can be reduced in the present inventionbecause operation is performed with an opposite electrode fixed at aconstant potential. A cost reduction can be achieved by usingtransistors with single polarity for transistors included in the pixelportion and a non-crystalline semiconductor film for semiconductorlayers of the transistors.

FIG. 330 shows a video camera, which includes a main body 3361, adisplay portion 3362, a chassis 3363, an external connection port 3364,a remote control receiving portion 3365, an image receiving portion3366, a battery 3367, an audio input portion 3368, an operation key3369, an eye piece portion 3360, and the like. Note that the pixeldescribed in any of Embodiment Modes 1 to 7 is used for the displayportion 3362. According to the present invention, variation in luminanceamong pixels or fluctuation in luminance of a pixel over time can besuppressed and a video camera including a display portion with a higherduty ratio and higher quality can be obtained. In addition, powerconsumption can be reduced in the present invention because operation isperformed with an opposite electrode fixed at a constant potential. Acost reduction can be achieved by using transistors with single polarityfor transistors included in the pixel portion and a non-crystallinesemiconductor film for semiconductor layers of the transistors.

FIG. 33H shows a mobile phone, which includes a main body 3371, achassis 3372, a display portion 3373, an audio input portion 3374, anaudio output portion 3375, an operation key 3376, an external connectionport 3377, an antenna 3378, and the like.

Note that the pixel described in any of Embodiment Modes 1 to 7 is usedfor the display portion 3373. According to the present invention,variation in luminance among pixels or fluctuation in luminance of apixel over time can be suppressed and a mobile phone including a displayportion with a higher duty ratio and higher quality can be obtained.

In addition, power consumption can be reduced in the present inventionbecause operation is performed with an opposite electrode fixed at aconstant potential. A cost reduction can be achieved by usingtransistors with single polarity for transistors included in the pixelportion and a non-crystalline semiconductor film for semiconductorlayers of the transistors.

As described above, the present invention can be applied to allelectronic devices.

Embodiment Mode 11

In this embodiment mode, an exemplary structure of a mobile phoneincluding the display device of the present invention in a displayportion is explained with reference to FIG. 34.

A display panel 3410 is incorporated in a housing 3400 so as to bedetachable. The shape and size of the housing 3400 can be appropriatelychanged in accordance with the size of the display panel 3410. Thehousing 3400 to which the display panel 3410 is fixed is fitted in aprinted circuit board 3401 and assembled as a module.

The display panel 3410 is connected to the printed circuit board 3401through an FPC 3411. The printed circuit board 3401 is provided with aspeaker 3402, a microphone 3403, a transmitting/receiving circuit 3404,and a signal processing circuit 3405 including a CPU, a controller, andthe like. Such a module, an input means 3406, and a buttery 3407 arecombined and stored in a chassis 3409 and a chassis 3412. Note that apixel portion of the display panel 3410 is arranged so as to be seenfrom a window formed in the chassis 3412.

In the display panel 3410, the pixel portion and a part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using transistors over thesame substrate, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed on an IC chip. The IC chip may be mountedon the display panel 3410 by COG (Chip On Glass). The IC chip mayalternatively be connected to a glass substrate using TAB (TapeAutomated Bonding) or a printed circuit board. Further, all of theperipheral driver circuits may be formed on an IC chip and the IC chipmay be mounted on a display panel by COO or the like.

Note that the pixel described in any of Embodiment Modes 1 to 7 is usedfor the pixel portion. According to the present invention, variation inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed and the display panel 3410 including a display portionwith a higher duty ratio and higher quality can be obtained. Inaddition, power consumption can be reduced in the present inventionbecause operation is performed with an opposite electrode fixed at aconstant potential. A cost reduction can be achieved by usingtransistors with single polarity for transistors included in the pixelportion and a non-crystalline semiconductor film for semiconductorlayers of the transistors.

The structure of a mobile phone described in this embodiment mode is oneexample, and the display device of the invention can be applied not onlyto the mobile phone having the above-described structure but also tomobile phones having various kinds of structures.

Embodiment Mode 12

In this embodiment mode, an EL module obtained by combining a displaypanic and a circuit board is explained with reference to FIGS. 35 and36.

As shown in FIG. 35, a display panel 3501 includes a pixel portion 3503,a scan line driver circuit 3504, and a signal line driver circuit 3505.Over a circuit board 3502, for example, a control circuit 3506, a signaldividing circuit 3507, and the like are formed. Note that the displaypanel 3501 and the circuit board 3502 are connected to each other by aconnection wiring 3508. As the connection wiring 3508, an FPC or thelike can be used.

In the display panel 3501, the pixel portion and a part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using transistors over thesame substrate, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed on an IC chip. The IC chip may be mountedon the display panel 3501 by COG (Chip On Glass). The IC chip mayalternatively be connected to a glass substrate using TAB (TapeAutomated Bonding) or a printed circuit board. Further, all of theperipheral driver circuits may be formed on an IC chip and the IC chipmay be mounted on a display panel by COG or the like.

Note that the pixel described in any of Embodiment Modes 1 to 7 is usedfor the pixel portion. According to the present invention, variation inluminance among pixels or fluctuation in luminance of a pixel over timecan be suppressed and the display panel 3501 with a higher duty ratioand higher quality can be obtained. In addition, power consumption canbe reduced in the present invention because operation is performed withan opposite electrode fixed at a constant potential. A cost reductioncan be achieved by using transistors with single polarity fortransistors included in the pixel portion and a non-crystallinesemiconductor film for semiconductor layers of the transistors.

An EL TV receiver can be completed with this EL module. FIG. 36 is ablock diagram showing the main structure of an EL TV receiver. A tuner3601 receives a video signal and an audio signal. The video signal isprocessed by a video signal amplifier circuit 3602, a video signalprocessing circuit 3603 for converting a signal outputted from the videosignal amplifier circuit 3602 into a color signal corresponding to eachcolor of red, green and blue, and a control circuit 3506 for convertingthe video signal into a signal which meets the input specification of adriver circuit. The control circuit 3506 outputs respective signals to ascan line side and a signal line side. In the case of performing adigital drive, it is possible to adopt a structure in which the signaldividing circuit 3507 is provided on the signal line side to supply aninput digital signal divided into m pieces.

The audio signal among the signals received by the tuner 3601 istransmitted to an audio signal amplifier circuit 3604, the output ofwhich is supplied to a speaker 3606 through an audio signal processingcircuit 3605. A control circuit 3607 receives control information of areceiving station (reception frequency) or sound volume from an inputportion 3608 and transmits signals to the tuner 3601 and the audiosignal processing circuit 3605.

By incorporating the EL module in FIG. 35 into the chassis 3301 of FIG.33A described in Embodiment Mode 10, a TV receiver can be completed.

Naturally, the present invention is not limited to the TV receiver, andcan be applied to various uses particularly as a large-sized displaymedium such as an information display board at a train station, anairport, or the like, or an advertisement display board on the street,as well as a monitor of a personal computer.

This application is based on Japanese Patent Application serial no.2005-349165 filed in Japan Patent Office on Dec. 2, 2005, the contentsof which are hereby incorporated by reference.

1. (canceled)
 2. A display device comprising: a pixel comprising: atransistor; a first switch; a second switch; a third switch; acapacitor; and a light emitting element, wherein a gate of thetransistor is electrically connected to a first line through the firstswitch, wherein the gate of the transistor is electrically connected toa first electrode of the capacitor, wherein one of a source and a drainof the transistor is electrically connected to a second electrode of thecapacitor, wherein the one of the source and the drain of the transistoris electrically connected to the light emitting element, wherein theother of the source and the drain of the transistor is electricallyconnected to a second line through the second switch, and wherein theone of the source and the drain of the transistor is electricallyconnected to a third line though the third switch.
 3. The display deviceaccording to claim 2, wherein the transistor is an n-channel transistor.4. The display device according to claim 2, wherein the transistorcomprises a semiconductor layer comprising silicon.
 5. The displaydevice according to claim 2, further comprising a fourth switch, whereinthe gate of the transistor is electrically connected to the second linethrough the fourth switch.
 6. A display device comprising: a pixelcomprising: a transistor; a first switch; a second switch; a thirdswitch; a capacitor; and a light emitting element, wherein a gate of thetransistor is electrically connected to a signal line through the firstswitch, wherein the gate of the transistor is electrically connected toa first electrode of the capacitor, wherein one of a source and a drainof the transistor is electrically connected to a second electrode of thecapacitor, wherein the one of the source and the drain of the transistoris electrically connected to the light emitting element, wherein theother of the source and the drain of the transistor is electricallyconnected to a power supply line through the second switch, and whereinthe one of the source and the drain of the transistor is electricallyconnected to a potential supply line though the third switch.
 7. Thedisplay device according to claim 6, wherein the transistor is ann-channel transistor.
 8. The display device according to claim 6,wherein the transistor comprises a semiconductor layer comprisingsilicon.
 9. The display device according to claim 6, further comprisinga fourth switch, wherein the gate of the transistor is electricallyconnected to the power supply line through the fourth switch.